add support for text/html content_type

[pawl-rs]

My endpoint returns text with Content-Type "text/html", and flex gives this error: "No content negotiation for this content type"

This PR adds support for the text/html Content-Type.

[pipermerriam]

@pawl-rs I'm not understanding how this is intended to work. It looks like you're just returning a string for the request.data field which is expected to be a dictionary (JSON) or maybe None or the empty string.