Add 'Peer-Address' property to connection metadata

Allows non-C/C++ based clients easy access to the peer's IP address via
zmq_msg_gets(&msg, "Peer-Address") instead of zmq_msg_get(&msg, ZMQ_SRCFD)
followed by calls to getpeername and getnameinfo

src/stream_engine.cpp

@@ -671,7 +671,7 @@ bool zmq::stream_engine_t::handshake ()
                 options.mechanism == ZMQ_GSSAPI? "GSSAPI":
                 "OTHER",
                 mechanism);
-            
+
             error (protocol_error);
             return false;
         }
@@ -806,6 +806,11 @@ void zmq::stream_engine_t::mechanism_ready ()
     properties_t properties;
     properties_t::const_iterator it;
 
+    //  If we have a peer_address, add it to metadata
+    if (!peer_address.empty()) {
+        properties.insert(std::make_pair("Peer-Address", peer_address));
+    }
+
     //  Add ZAP properties.
     const properties_t& zap_properties = mechanism->get_zap_properties ();
     it = zap_properties.begin ();