Add support for Webmock #126

When making assertions within Webmock on a request body, if the assertion fails, Webmock will spit out a diff of the changes. This is more or less documented here: https://github.com/bblimke/webmock/blob/master/spec/unit/request_body_diff_spec.rb. This diff can get quite confusing, so we should figure out a way to tap into this.