This change addresses part of issue 4336.

Change endheaders() to take an optional message_body argument
that is sent along with the headers.  Change xmlrpclib and
httplib's other methods to use this new interface.

It is more efficient to make a single send() call, which should
get the entire client request into one packet (assuming it is
smaller than the MTU) and will avoid the long pause for delayed
ack following timeout.

Also:
- Add a comment about the buffer size for makefile().
- Extract _set_content_length() method and fix whitespace issues there.