OFHTTPClient: Fix -[close] behaviour.

-[OFHTTPClient close] now closes the connection, no matter whether a
reply still exists. -[OFHTTPRequestReply close] only disposes of the
reference to the socket now, so that if the OFHTTPClient does not exist
anymore the socket is closed, but not if the OFHTTPClient is still alive
and in keep-alive mode. This is important so that closing a single reply
does not close the client even though keep-alive was used.