add a way to tell if the content type has changed on the response

In Pyramid our renderers attempt to change the content_type of the response object to a sane default if and only if the user has not changed it themselves. We've traditionally used if response.content_type == response.default_content_type where the default_content_type == 'text/html' however in some cases we've found the user attempting to override a renderer like the string renderer to return text/html. In this case the user sets response.content_type = 'text/html' and the string renderer sees no changes and overrides it to text/plain.

TLDR A way to tell if the content_type was set.

Related: https://github.com/Pylons/pyramid/pull/1565 and https://github.com/Pylons/pyramid/issues/1344