BUG/MINOR: http_client: Don't add input data on an empty request buffer

BUG/MINOR: http_client: Don't add input data on an empty request buffer

When compiled in debug mode, a BUG_ON triggers an error when the payload is
fully transfered from the http-client buffer to the request channel
buffer. In fact, when channel_add_input() is called, the request buffer is
empty. So an error is reported when those data are directly forwarded,
because we try to add some output data on a buffer with no data.

To fix the bug, we must be sure to call channel_add_input() after the data
transfer.

The bug was introduced by the commit ccc7ee45f ("MINOR: httpclient: enable
request buffering"). So, this patch must be backported if the above commit
is backported.