Make _test_update_resource honor format

Closes-Bug: #1217759

Partial-Bug: #1217791
Due to the Related-Bug: #1217791, MyComparator does not work
in some cases. We need to use a workaround that serialized body
to mocked self.client.httpclient.request. It is already adopted
in _test_create_resourse and I borrow it.

Change-Id: I746d272bc7ba8542c29e47823f707a761dddb0b1
Merge "Make _test_update_resource honor format"