Use "Content-Type" from request/response headers

It seems unnecessary to use "request_content_type" and "response_content_type"
when those data are already included in the respective headers dictionaries.

I don't think it's useful to have these extra keys be part of the expected JSON
payload. Indeed, http_spec does not generate them, although RAD does.

This change continues to work with RAD, because it includes the Content-Type
header, but also works with http_spec.

I could have changed http_spec to include these keys, but I think this is a
better fix.