[BUG] http: update the header list's tail when removing the last header

[BUG] http: update the header list's tail when removing the last header

Stefan Behte reported a strange case where depending on the position of
the Connection header in the header list, some headers added after it
were or were not usable in "balance hdr()". The reason is that when the
last header is removed, the list's tail was not updated, so any header
added after that one was not visible from the list.

This fix must be backported to 1.4 and possibly 1.3.