Ajax: Support null as success functions in jQuery.get

According to the docs, one can use `null` as a success function in `jQuery.get`
of `jQuery.post` so the following:

```js
await jQuery.get( "https://httpbin.org/json", null, "text" )
```

should get the text result. However, this shortcut hasn't been working so far.

Fixes gh-4989
Closes gh-5139

Author: mgol

src/ajax.js

@@ -846,8 +846,9 @@ jQuery.extend( {
 jQuery.each( [ "get", "post" ], function( _i, method ) {
 	jQuery[ method ] = function( url, data, callback, type ) {
 
-		// Shift arguments if data argument was omitted
-		if ( typeof data === "function" ) {
+		// Shift arguments if data argument was omitted.
+		// Handle the null callback placeholder.
+		if ( typeof data === "function" || data === null ) {
 			type = type || callback;
 			callback = data;
 			data = undefined;

src/ajax/xhr.js

@@ -15,7 +15,6 @@ var xhrSuccessStatus = {
 jQuery.ajaxTransport( function( options ) {
 	var callback;
 
-	// Cross domain only allowed if supported through XMLHttpRequest
 	return {
 		send: function( headers, complete ) {
 			var i,

test/data/mock.php

@@ -95,9 +95,9 @@ protected function json( $req ) {
 		}
 
 		if ( isset( $req->query['array'] ) ) {
-			echo '[ {"name": "John", "age": 21}, {"name": "Peter", "age": 25 } ]';
+			echo '[{"name":"John","age":21},{"name":"Peter","age":25}]';
 		} else {
-			echo '{ "data": {"lang": "en", "length": 25} }';
+			echo '{"data":{"lang":"en","length":25}}';
 		}
 	}
 
@@ -112,8 +112,8 @@ protected function jsonp( $req ) {
 			$callback = $_POST['callback'];
 		}
 		$json = isset( $req->query['array'] ) ?
-			'[ { "name": "John", "age": 21 }, { "name": "Peter", "age": 25 } ]' :
-			'{ "data": { "lang": "en", "length": 25 } }';
+			'[{"name":"John","age":21},{"name":"Peter","age":25}]' :
+			'{"data":{"lang":"en","length":25}}';
 		echo cleanCallback( $callback ) . '(' . $json . ')';
 	}
 

test/unit/ajax.js

@@ -2593,6 +2593,26 @@ if ( typeof window.ArrayBuffer === "undefined" || typeof new XMLHttpRequest().re
 		} );
 	} );
 
+	QUnit.test( "jQuery.get( String, null-ish, String ) - dataType with null callback (gh-4989)",
+			function( assert ) {
+		assert.expect( 2 );
+		var done = assert.async( 2 );
+
+		jQuery.get( url( "mock.php?action=json&header" ), null, "json" )
+			.then( function( json ) {
+				assert.deepEqual( json, { data: { lang: "en", length: 25 } },
+					"`dataType: \"json\"` applied with a `null` callback" );
+				done();
+			} );
+
+		jQuery.get( url( "mock.php?action=json&header" ), null, "text" )
+			.then( function( text ) {
+				assert.strictEqual( text, "{\"data\":{\"lang\":\"en\",\"length\":25}}",
+					"`dataType: \"text\"` applied with a `null` callback" );
+				done();
+			} );
+	} );
+
 //----------- jQuery.getJSON()
 
 	QUnit.test( "jQuery.getJSON( String, Hash, Function ) - JSON array", function( assert ) {