Better error message when passing non-string to custom method

Raises a slightly more helpful error message when passing a non-string
to a custom method (currently, it reads "no implicit conversion of Hash
into String", which is terrible).

This a partial remediation for the problem encountered in #809.

lib/stripe/api_resource.rb

@@ -70,6 +70,11 @@ module Stripe
       end
       http_path ||= name.to_s
       define_singleton_method(name) do |id, params = {}, opts = {}|
+        unless id.is_a?(String)
+          raise ArgumentError,
+                "id should be a string representing the ID of an API resource"
+        end
+
         url = "#{resource_url}/#{CGI.escape(id)}/#{CGI.escape(http_path)}"
         resp, opts = request(http_verb, url, params, opts)
         Util.convert_to_stripe_object(resp.data, opts)

test/stripe/api_resource_test.rb

@@ -4,10 +4,31 @@ require ::File.expand_path("../test_helper", __dir__)
 
 module Stripe
   class ApiResourceTest < Test::Unit::TestCase
+    class CustomMethodAPIResource < APIResource
+      OBJECT_NAME = "custom_method".freeze
+      custom_method :my_method, http_verb: :post
+    end
+
     class NestedTestAPIResource < APIResource
       save_nested_resource :external_account
     end
 
+    context ".custom_method" do
+      should "call to an RPC-style method" do
+        stub_request(:post, "#{Stripe.api_base}/v1/custom_methods/ch_123/my_method")
+          .to_return(body: JSON.generate({}))
+        CustomMethodAPIResource.my_method("ch_123")
+      end
+
+      should "raise an error if a non-ID is passed" do
+        e = assert_raises ArgumentError do
+          CustomMethodAPIResource.my_method(id: "ch_123")
+        end
+        assert_equal "id should be a string representing the ID of an API resource",
+                     e.message
+      end
+    end
+
     context ".save_nested_resource" do
       should "can have a scalar set" do
         r = NestedTestAPIResource.new("test_resource")