Rails actionpack responder preserves :location => nil, AM::S should too

In a case where we want to `respond_with thing, location: nil`, Rails will preserve that location and not attempt to find `thing_url` on create, for example.

This turns out to be a bug in the way `display` in the responder is overridden. To remedy this, we can do what Rails does (which is a bit obfuscated, IMO) which is to make sure to merge the original responder options into the `given_options` to preserve location before passing it to `render`.