Always add special-cased 'Content-Type' header to request.

Because 'Content-Type' needs special-cased to hand inside the request body's 'TypedOutput' we were erroneously omitting the header if no body was present. In the case of no body, we can just add it like any other header.
Merge pull request #553 from square/jw/content-type-header-no-body

Always add special-cased 'Content-Type' header to request.