Expose xhr.withCredentials with correct default

Fixes driverdan#71
Replaces driverdan#84

Author: kumavis

lib/XMLHttpRequest.js

@@ -110,6 +110,10 @@ exports.XMLHttpRequest = function() {
   this.responseXML = "";
   this.status = null;
   this.statusText = null;
+  
+  // Whether cross-site Access-Control requests should be made using
+  // credentials such as cookies or authorization headers
+  this.withCredentials = false;
 
   /**
    * Private methods
@@ -363,7 +367,8 @@ exports.XMLHttpRequest = function() {
       path: uri,
       method: settings.method,
       headers: headers,
-      agent: false
+      agent: false,
+      withCredentials: self.withCredentials
     };
 
     // Reset error flag
@@ -399,7 +404,8 @@ exports.XMLHttpRequest = function() {
             port: url.port,
             path: url.path,
             method: response.statusCode === 303 ? "GET" : settings.method,
-            headers: headers
+            headers: headers,
+            withCredentials: self.withCredentials
           };
 
           // Issue the new request