When IE6 & IE7 serve from the cache in async mode, resolves the request...

When IE6 & IE7 serve from the cache in async mode, resolves the request asynchronously. Fixes #11778.

src/ajax/xhr.js

@@ -190,11 +190,13 @@ if ( jQuery.support.ajax ) {
 						}
 					};
 
-					// if we're in sync mode or it's in cache
-					// and has been retrieved directly (IE6 & IE7)
-					// we need to manually fire the callback
-					if ( !s.async || xhr.readyState === 4 ) {
+					if ( !s.async ) {
+						// if we're in sync mode we fire the callback
 						callback();
+					} else if ( xhr.readyState === 4 ) {
+						// (IE6 & IE7) if it's in cache and has been
+						// retrieved directly we need to fire the callback
+						setTimeout( callback, 0 );
 					} else {
 						handle = ++xhrId;
 						if ( xhrOnUnloadAbort ) {