[BUG] http: trim any excess buffer data when recycling a connection

[BUG] http: trim any excess buffer data when recycling a connection

We must trim any excess data from the response buffer when recycling
a keep-alive connection, because we may have blocked an invalid response
from a server that we don't want to accidentely forward once we disable
the analysers, nor do we want those data to come along with next response.
A typical example of such data would be from a buggy server responding to
a HEAD with some data, or sending more than the advertised content-length.