BUG/MINOR: http-client: Don't forget to commit changes on HTX message

BUG/MINOR: http-client: Don't forget to commit changes on HTX message

In the http-client I/O handler, HTX request and response are loaded from the
channels buffer. Some changes are preformed in these messages. So, we must
take care to commit changes into the underlying buffer by calling
htx_to_buf().

It is especially important when the HTX message becoms empty to be able to
quickly release the buffer.

This patch should be backported as far as 2.6.