Fixes #8146 by introducing the xhrFields option with is a map of...

Fixes #8146 by introducing the xhrFields option with is a map of fieldName/fieldValue to set on the native xhr. Can be used to set withCredentials to true for cross-domain requests if needed.

src/ajax/xhr.js

@@ -82,7 +82,15 @@ if ( jQuery.support.ajax ) {
 
 					// Get a new xhr
 					var xhr = s.xhr(),
-						handle;
+						handle,
+						i;
+
+					// Apply custom fields if provided
+					if ( s.xhrFields ) {
+						for ( i in s.xhrFields ) {
+							xhr[ i ] = s.xhrFields[ i ];
+						}
+					}
 
 					// Open the socket
 					// Passing null username, generates a login popup on Opera (#2865)