BUG/MINOR: httpclient: Count metadata in size to transfer via htx_xfer_blks()

BUG/MINOR: httpclient: Count metadata in size to transfer via htx_xfer_blks()

When HTX blocks are transfer from the HTTP client context to the request
channel, via htx_xfer_blks() function, the metadata must also be counted, in
addition to the data size. Otherwise, expected payload size will not be
copied because the metadata of an HTX block (8 bytes) will be reserved. And
if the payload size is lower than 8 bytes, nothing will be copied. Thus only
a zero-copy will be able to copy the payload.

This issue is 2.6-specific, no backport is needed.