Fixed a bug in frame's move assignment operator.

The zmq_msg_init() at the beginning created a new message overwriting
the handle of the old message. But this old message was not closed
leaving a memory leak behind. Instead of closing the existing handle
explicitly we can simply remove the init call because zmq_msg_move()
does the closing of the old message on its own.

see zeromq#204

src/zmqpp/frame.cpp

@@ -81,7 +81,6 @@ frame::frame(frame&& other)
 
 frame& frame::operator=(frame&& other)
 {
-	zmq_msg_init( &_msg );
 	zmq_msg_move( &_msg, &other._msg );
 	std::swap( _sent, other._sent );