Fix XmppSignalStrategy to return correct result from SendStanza().
Previously XmppSignalStrategy::SendStanza() was returning true even
after write has failed on the socket. Failed socket write causes the
XMPP connection to be closed. When sending host offline reason that
results in HeartbeatSender being destroyed together with the IqSender
that was used to send the message, which was causing the crash in the
linked bug. Now IqSender::SendIq() can handle properly the case when