MEDIUM: httpclient: add a Content-Length when the payload is known

MEDIUM: httpclient: add a Content-Length when the payload is known

This introduce a change of behavior in the httpclient API. When
generating a request with a payload buffer, the size of the buffer
payload is known and does not need to be streamed in chunks.

This patch force to sends payload buffer using a Content-Length header
in the request, however the behavior does not change if a callback is
still used instead of a buffer.