BUG/MINOR: channel: inject output data at the end of output

BUG/MINOR: channel: inject output data at the end of output

In co_inject(), data must be inserted at the end of output, not the end of
input. For the record, this function does not take care of input data which are
supposed to not exist. But the caller may reset input data after or before the
call. It is its own choice.

This bug, among other effects, is visible when a redirect is performed on
the response path, on legacy HTTP mode (so for HAProxy < 2.1). The redirect
response is appended after the server response when it should overwrite it.

Thanks to Kevin Zhu <ip0tcp@gmail.com> to report the bug. It must be backported
as far as 1.9.