lib: fix behavior with util.inspect

If one tried to print the result of `require('metarhia-jstp')` in
general or the `Connection` class itself to console, it resulted in
invocation of internal logic of the `Connection` and crashing with
an exception.

A similar issue had been fixed in
f7c393b and
76e4323 but was introduced again in
4a7941a.

PR-URL: #72

lib/connection.js

@@ -13,6 +13,11 @@ module.exports = Connection;
 
 let nextConnectionId = 0;
 
+// Mapping of packet types to handler function names (forward declaration, see
+// definition below the Connection class).
+//
+let PACKET_HANDLERS;  // eslint-disable-line prefer-const
+
 // JSTP connection class
 //   transport - an abstract socket
 //   server - JSTP server instance, used only for server-side parts
@@ -322,7 +327,7 @@ Connection.prototype._processPacket = function(packet) {
     return;
   }
 
-  const handler = Connection.PACKET_HANDLERS[kind];
+  const handler = PACKET_HANDLERS[kind];
   if (handler) {
     handler.call(this, packet, keys);
   } else {
@@ -617,9 +622,7 @@ Connection.prototype._emitPacketEvent = function(kind, packet, packetId, args) {
   this.emit(kind, eventArgs);
 };
 
-// Mapping of packet types to handler function names
-//
-Connection.PACKET_HANDLERS = {
+PACKET_HANDLERS = {
   handshake: Connection.prototype._processHandshakePacket,
   call:      Connection.prototype._processCallPacket,
   callback:  Connection.prototype._processCallbackPacket,