Add support for application/pdf content type #607

Is your feature request related to a problem? Please describe.
I have a 3rd party schema describing an API which returns a PDF file with content application/pdf

Describe the solution you'd like
I'd like either to add the following line to

"application/pdf": "response.content"

or some interface to add that mapping myself.

Describe alternatives you've considered
I'm currently patching _SOURCE_BY_CONTENT_TYPE to workaround this.