Nicer error when specifying non-nil non-string opt value

Previously, if you specified a non-nil non-string opt value, like a
symbol for `idempotency_key`, you'd get a pretty user-unfriendly error
from `Net::HTTP`:

```
/Users/brandur/.rbenv/versions/2.4.5/lib/ruby/2.4.0/net/http/header.rb:21:in `block in initialize_http_header': undefined method `strip' for :foo:Symbol (NoMethodError)
```

Here, we introduce a new argument error that makes it a little easier
for someone to read. The impetus for the change is that we had an
internal product quality report where someone ran into this and was
confused.

I'm pretty sure this change is backward compatible because `Net::HTTP`
would call `strip` on anything that was passed in as a value, and
generally just strings would support that. There may be some other less
common data type that was accidentally compatible that someone was
using, but that case should be quite unusual.

lib/stripe/api_operations/request.rb

@@ -8,6 +8,8 @@ def request(method, url, params = {}, opts = {})
           warn_on_opts_in_params(params)
 
           opts = Util.normalize_opts(opts)
+          error_on_non_string_user_opts(opts)
+
           opts[:client] ||= StripeClient.active_client
 
           headers = opts.clone
@@ -31,10 +33,24 @@ def request(method, url, params = {}, opts = {})
           [resp, opts_to_persist]
         end
 
+        private def error_on_non_string_user_opts(opts)
+          Util::OPTS_USER_SPECIFIED.each do |opt|
+            next unless opts.key?(opt)
+
+            val = opts[opt]
+            next if val.nil?
+            next if val.is_a?(String)
+
+            raise ArgumentError,
+                  "request option '#{opt}' should be a string value " \
+                    "(was a #{val.class})"
+          end
+        end
+
         private def warn_on_opts_in_params(params)
           Util::OPTS_USER_SPECIFIED.each do |opt|
             if params.key?(opt)
-              warn("WARNING: #{opt} should be in opts instead of params.")
+              warn("WARNING: '#{opt}' should be in opts instead of params.")
             end
           end
         end

test/stripe/api_resource_test.rb

@@ -227,6 +227,23 @@ class NestedTestAPIResource < APIResource
         end
       end
 
+      should "error if a user-specified opt is given a non-nil non-string value" do
+        stub_request(:post, "#{Stripe.api_base}/v1/charges")
+          .to_return(body: JSON.generate(charge_fixture))
+
+        # Works fine if not included or a string.
+        Stripe::Charge.create({ amount: 100, currency: "usd" }, {})
+        Stripe::Charge.create({ amount: 100, currency: "usd" }, idempotency_key: "12345")
+
+        # Errors on a non-string.
+        e = assert_raises(ArgumentError) do
+          Stripe::Charge.create({ amount: 100, currency: "usd" }, idempotency_key: :foo)
+        end
+        assert_equal "request option 'idempotency_key' should be a string value " \
+                       "(was a Symbol)",
+                     e.message
+      end
+
       should "requesting with a unicode ID should result in a request" do
         stub_request(:get, "#{Stripe.api_base}/v1/customers/%E2%98%83")
           .to_return(body: JSON.generate(make_missing_id_error), status: 404)