Tests: Add tests for jQuery.get( String, null-ish, null-ish, String )

Also, fix `mock.php` formatting to not fail the
`jQuery.get( String, null, String )` test in PHP mode.

Ref jquerygh-4989
Ref jquery/api.jquery.com#1208

Author: mgol

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}}';
 		}
 	}
 

test/unit/ajax.js

@@ -2782,7 +2782,7 @@ if ( typeof window.ArrayBuffer === "undefined" || typeof new XMLHttpRequest().re
 		} );
 	} );
 
-	QUnit.test( "jQuery.get( String, null-ish, String ) - dataType with null callback (gh-4989)",
+	QUnit.test( "jQuery.get( String, null, String ) - dataType with null callback (gh-4989)",
 			function( assert ) {
 		assert.expect( 2 );
 		var done = assert.async( 2 );
@@ -2802,6 +2802,37 @@ if ( typeof window.ArrayBuffer === "undefined" || typeof new XMLHttpRequest().re
 			} );
 	} );
 
+	QUnit.test( "jQuery.get( String, null-ish, null-ish, String ) - dataType with null/undefined  data & callback",
+			function( assert ) {
+		assert.expect( 8 );
+		var done = assert.async( 8 );
+
+		[
+			{ data: null, success: null },
+			{ data: null, success: undefined },
+			{ data: undefined, success: null },
+			{ data: undefined, success: undefined }
+		].forEach( function( options ) {
+			var data = options.data,
+				success = options.success;
+			jQuery.get( url( "mock.php?action=json&header" ), data, success, "json" )
+				.then( function( json ) {
+					assert.deepEqual( json, { data: { lang: "en", length: 25 } },
+						"`dataType: \"json\"` applied with `" + data + "` data & `" +
+						success + "` success 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 `" + data + "` data & `" +
+						success + "` success callback" );
+					done();
+				} );
+		} );
+	} );
+
 //----------- jQuery.getJSON()
 
 	QUnit.test( "jQuery.getJSON( String, Hash, Function ) - JSON array", function( assert ) {