Flag Account#external_account into save_with_parent system

In #433, we built a framework under which subresources are usually not
persisted, but in certain cases they can be. At the time,
`Customer#source` was the only field that I knew about that had to be
flagged into it.

Issue #456 has raised that we also be doing `Account#external_account`.
This patch adds support for that.

Fixes #456.

lib/stripe/account.rb

@@ -28,6 +28,23 @@ module Stripe
       super(id, opts)
     end
 
+    # Set or replace an account's external account.
+    def external_account=(value)
+      super
+
+      # The parent setter will perform certain useful operations like
+      # converting to an APIResource if appropriate.
+      value = self.external_account
+
+      # Note that external_account may be a card, but could also be a tokenized
+      # card's ID (which is a string), and so we check its type here.
+      if value.is_a?(APIResource)
+        value.save_with_parent = true
+      end
+
+      value
+    end
+
     def reject(params={}, opts={})
       opts = Util.normalize_opts(opts)
       response, opts = request(:post, resource_url + '/reject', params, opts)

test/stripe/account_test.rb

@@ -289,5 +289,19 @@ module Stripe
       }
       assert_equal(expected, obj.serialize_params)
     end
+
+    should "can have a token source set" do
+      a = Stripe::Account.new("test_account")
+      a.external_account = "tok_123"
+      assert_equal "tok_123", a.external_account
+    end
+
+    should "set a flag if given an object source" do
+      a = Stripe::Account.new("test_account")
+      a.external_account = {
+        :object => 'card'
+      }
+      assert_equal true, a.external_account.save_with_parent
+    end
   end
 end