Ajax: Support an alternative completeCallback API for transports

Apart from the existing API:

```js
function( status, statusText, responses, headers ) {}
```
a new API is now available:
```js
function( { status, statusText, responses, headers } ) {}
```

This makes it possible to add new parameters in the future without relying on
their order among parameters and being able to provide them selectively.

Ref gh-4405

src/ajax.js

@@ -713,7 +713,18 @@ jQuery.extend( {
 
 		// Callback for when everything is done
 		function done( status, nativeStatusText, responses, headers ) {
-			var isSuccess, success, error, response, modified,
+			var isSuccess, success, error, response, modified, statusText;
+
+			if ( typeof status === "object" ) {
+
+				// The new, object-based API
+				nativeStatusText = status.statusText;
+				responses = status.responses;
+				headers = status.headers;
+
+				status = status.status;
+			}
+
 			statusText = nativeStatusText;
 
 			// Ignore repeat invocations

src/ajax/xhr.js

@@ -63,23 +63,23 @@ jQuery.ajaxTransport( function( options ) {
 						if ( type === "abort" ) {
 							xhr.abort();
 						} else if ( type === "error" ) {
-							complete(
+							complete( {
 
 								// File: protocol always yields status 0; see trac-8605, trac-14207
-								xhr.status,
-								xhr.statusText
-							);
+								status: xhr.status,
+								statusText: xhr.statusText
+							} );
 						} else {
-							complete(
-								xhrSuccessStatus[ xhr.status ] || xhr.status,
-								xhr.statusText,
+							complete( {
+								status: xhrSuccessStatus[ xhr.status ] || xhr.status,
+								statusText: xhr.statusText,
 
 								// For XHR2 non-text, let the caller handle it (gh-2498)
-								( xhr.responseType || "text" ) === "text" ?
+								responses: ( xhr.responseType || "text" ) === "text" ?
 									{ text: xhr.responseText } :
 									{ binary: xhr.response },
-								xhr.getAllResponseHeaders()
-							);
+								headers: xhr.getAllResponseHeaders()
+							} );
 						}
 					}
 				};