LIBZMQ-497 there could be unsent bytes in encoder

When we send a large message, the message can be splitted into two chunks.
One is in the encoder buffer and the other is the zero-copy pointer.
The session could get the term before the last chunk is sent.

src/i_encoder.hpp

@@ -47,6 +47,7 @@ namespace zmq
         virtual void get_data (unsigned char **data_, size_t *size_,
             int *offset_ = NULL) = 0;
 
+        virtual bool has_data () = 0;
     };
 
 }