Allow existing Socket object to be injected.

lib/Connection.js

@@ -47,6 +47,7 @@ function Connection(config) {
   config || (config = {});
 
   this._config = {
+    socket: config.socket,
     host: config.host || 'localhost',
     port: config.port || 143,
     tls: config.tls,
@@ -63,7 +64,7 @@ function Connection(config) {
                 : true)
   };
 
-  this._sock = undefined;
+  this._sock = config.socket || undefined;
   this._tagcount = 0;
   this._tmrConn = undefined;
   this._tmrKeepalive = undefined;
@@ -83,7 +84,7 @@ inherits(Connection, EventEmitter);
 Connection.prototype.connect = function() {
   var config = this._config, self = this, socket, parser, tlsOptions;
 
-  socket = new Socket();
+  socket = config.socket || new Socket();
   socket.setKeepAlive(true);
   socket.setTimeout(0);
   this._sock = undefined;