Fix LIBZMQ-576 - Revert "Merge pull request #510 from miniway/master"

This reverts commit f27eb67e1a, reversing
changes made to a3ae0d4c16.

https://zeromq.jira.com/browse/LIBZMQ-576

Conflicts:
	src/stream_engine.cpp

Conflicts were around additional defaults to the constructor after the
'terminating' default.  The additional defaults were left alone, and
the 'terminating' default was removed.

src/i_encoder.hpp

@@ -43,7 +43,6 @@ namespace zmq
         //  Load a new message into encoder.
         virtual void load_msg (msg_t *msg_) = 0;
 
-        virtual bool has_data () = 0;
     };
 
 }