Merge pull request #552 from square/jw/wtf-dot-net

Add trailing CRLF to multipart.
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.