MINOR: httpclient: enable request buffering

MINOR: httpclient: enable request buffering

The request buffering is required for doing l7 retry. The IO handler of
the httpclient need to be rework for that.

This patch change the IO handler so it copies partially the data instead
of swapping buffer. This is needed because the b_xfer won't never work
if the destination buffer is not empty, which is the case when
buffering.