Add support for the Account Link resource

Also fixed a test as the latest openapi spec removed support for it. It's
now gated. Leaving in the library though to avoid a major version

src/Stripe.net/Entities/AccountLinks/AccountLink.cs

@@ -0,0 +1,24 @@
+namespace Stripe
+{
+    using System;
+    using System.Collections.Generic;
+    using Newtonsoft.Json;
+    using Stripe.Infrastructure;
+
+    public class AccountLink : StripeEntity, IHasObject
+    {
+        [JsonProperty("object")]
+        public string Object { get; set; }
+
+        [JsonProperty("created")]
+        [JsonConverter(typeof(DateTimeConverter))]
+        public DateTime? Created { get; set; }
+
+        [JsonProperty("expires_at")]
+        [JsonConverter(typeof(DateTimeConverter))]
+        public DateTime? ExpiresAt { get; set; }
+
+        [JsonProperty("url")]
+        public string Url { get; set; }
+     }
+}

src/Stripe.net/Services/AccountLinks/AccountLinkCreateOptions.cs

@@ -0,0 +1,25 @@
+namespace Stripe
+{
+    using System;
+    using System.Collections.Generic;
+    using Newtonsoft.Json;
+    using Stripe.Infrastructure;
+
+    public class AccountLinkCreateOptions : BaseOptions
+    {
+        [JsonProperty("account")]
+        public string AccountId { get; set; }
+
+        [JsonProperty("collect")]
+        public string Collect { get; set; }
+
+        [JsonProperty("failure_url")]
+        public string FailureUrl { get; set; }
+
+        [JsonProperty("success_url")]
+        public string SuccessUrl { get; set; }
+
+        [JsonProperty("type")]
+        public string Type { get; set; }
+    }
+}

src/Stripe.net/Services/AccountLinks/AccountLinkService.cs

@@ -0,0 +1,34 @@
+namespace Stripe
+{
+    using System.Collections.Generic;
+    using System.Net;
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Stripe.Infrastructure;
+
+    public class AccountLinkService : Service<AccountLink>,
+        ICreatable<AccountLink, AccountLinkCreateOptions>
+    {
+        public AccountLinkService()
+            : base(null)
+        {
+        }
+
+        public AccountLinkService(string apiKey)
+            : base(apiKey)
+        {
+        }
+
+        public override string BasePath => "/account_links";
+
+        public virtual AccountLink Create(AccountLinkCreateOptions options, RequestOptions requestOptions = null)
+        {
+            return this.CreateEntity(options, requestOptions);
+        }
+
+        public virtual Task<AccountLink> CreateAsync(AccountLinkCreateOptions options, RequestOptions requestOptions = null, CancellationToken cancellationToken = default(CancellationToken))
+        {
+            return this.CreateEntityAsync(options, requestOptions, cancellationToken);
+        }
+    }
+}

src/StripeTests/Entities/Coupons/AccountLinkTest.cs

@@ -0,0 +1,19 @@
+namespace StripeTests
+{
+    using Newtonsoft.Json;
+    using Stripe;
+    using Xunit;
+
+    public class AccountLinkTest : BaseStripeTest
+    {
+        [Fact]
+        public void Deserialize()
+        {
+            var json = GetResourceAsString("api_fixtures.account_link.json");
+            var accountLink = JsonConvert.DeserializeObject<AccountLink>(json);
+            Assert.NotNull(accountLink);
+            Assert.IsType<AccountLink>(accountLink);
+            Assert.Equal("account_link", accountLink.Object);
+        }
+    }
+}

src/StripeTests/Resources/api_fixtures/account_link.json

@@ -0,0 +1,6 @@
+{
+  "object": "account_link",
+  "created": 1544220862,
+  "expires_at": 1544221162,
+  "url": "https://stripe.com/hv/12345"
+}

src/StripeTests/Services/AccountLinks/AccountLinkServiceTest.cs

@@ -0,0 +1,47 @@
+namespace StripeTests
+{
+    using System.Collections.Generic;
+    using System.Net.Http;
+    using System.Threading.Tasks;
+
+    using Stripe;
+    using Xunit;
+
+    public class AccountLinkServiceTest : BaseStripeTest
+    {
+        private AccountLinkService service;
+        private AccountLinkCreateOptions createOptions;
+
+        public AccountLinkServiceTest()
+        {
+            this.service = new AccountLinkService();
+
+            this.createOptions = new AccountLinkCreateOptions()
+            {
+                AccountId = "acct_123",
+                Collect = "eventually_due",
+                FailureUrl = "https://stripe.com/failure",
+                SuccessUrl = "https://stripe.com/success",
+                Type = "custom_account_verification",
+            };
+        }
+
+        [Fact]
+        public void Create()
+        {
+            var accountLink = this.service.Create(this.createOptions);
+            this.AssertRequest(HttpMethod.Post, "/v1/account_links");
+            Assert.NotNull(accountLink);
+            Assert.Equal("account_link", accountLink.Object);
+        }
+
+        [Fact]
+        public async Task CreateAsync()
+        {
+            var accountLink = await this.service.CreateAsync(this.createOptions);
+            this.AssertRequest(HttpMethod.Post, "/v1/account_links");
+            Assert.NotNull(accountLink);
+            Assert.Equal("account_link", accountLink.Object);
+        }
+    }
+}

src/StripeTests/Services/Terminal/Locations/LocationServiceTest.cs

@@ -38,7 +38,6 @@ public LocationServiceTest()
 
             this.listOptions = new LocationListOptions
             {
-                OperatorAccount = "acct_123",
             };
 
             this.updateOptions = new LocationUpdateOptions