bridge: Don't send Content-Length header for non-POST requests
bridge: Don't send Content-Length header for non-POST requests

When sending a non-POST request, if no data has been sent in the
channel, then don't send a Content-Length header in the HTTP request.

This is in keeping with the HTTP RFC, which says we should not send
Content-Length without data for methods whose semantics do not include
a body.

Closes #1832
Reviewed-by: Marius Vollmer <bb11c6e942c5cc765df5f04da146b3198127a7a3@redhat.com>

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