Accepts content_type kwarg for the json_view decorator.

I want to use more specific content-types than `application/json`, things like `application/vnd.com.myservice.v2+json`. I could just override it in the return values of the functions, but I think specifying it as a decorator parameter would be nice. It's a huge diff but if you ignore whitespaces changes caused by one extra level of indentation, it was a tiny change. I had to change line breaks in a few places just keep flake8 happy about the line length.