Fixing `layout: :none` in controller rendering

Regarding https://github.com/phoenixframework/phoenix/commit/bc2c2899fff7fee7f066d0fecf85a2a3c8dfdfca, `layout: :none` wasn't working. `prepare_for_render/4` wasn't paying attention to the user-provided `assigns`. I changed the default to be assigned by `assign_layout/2` rather than fetched by `layout/1`, hope that's ok.

Foregoing an integration test per https://github.com/phoenixframework/phoenix/pull/178#issuecomment-48829583