Use a static instance for default HttpClient

[ob-stripe]

r? @brandur-stripe @richardm-stripe
cc @stripe/api-libraries

Update Stripe.SystemNetHttpClient to use a static instance of System.Net.HttpClient when no HTTP client is passed to the constructor. This means that multiple clients will share the same instance of System.Net.HttpClient.

This is what Microsoft advises in https://docs.microsoft.com/en-us/aspnet/web-api/overview/advanced/calling-a-web-api-from-a-net-client#create-and-initialize-httpclient:

HttpClient is intended to be instantiated once and reused throughout the life of an application.

This is thread-safe. The only method we're using is SendAsync, which is explicitly listed as thread-safe in the documentation: https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient?redirectedfrom=MSDN&view=netframework-4.8#remarks.

The default instance is wrapped in a Lazy<> initializer, so it's only initialized the first time it's needed (to avoid initializing it if it's never used, e.g. because the user is always supplying their own instance).

It's difficult to write a test for this because httpClient is private, but I temporarily bumped its visibility and verified that this test passed:

var client1 = new SystemNetHttpClient();
var client2 = new SystemNetHttpClient();

Assert.NotSame(client1, client2);
Assert.Same(client1.HttpClient, client2.HttpClient);

Fixes #1918.

[brandur-stripe]

It's difficult to write a test for this because httpClient is private, but I temporarily bumped its visibility and verified that this test passed:

Nice extra rigor :)

LGTM.

[ob-stripe]

Thanks Brandur!