Widget: Avoid adding extra spaces to the result of this._classes(...)

Closes gh-1372

Author: Gabriel Schulhof

tests/unit/widget/widget_core.js

@@ -608,17 +608,20 @@ test( ".option() - deep option setter", function() {
 });
 
 test( "_classes", function(){
-	expect( 3 );
+	expect( 4 );
 	$.widget( "ui.testWidget", {
 		options: {
 			classes: {
 				"test": "class1 class2",
+				"testEmpty": "",
 				"test2": "class3"
 			}
 		},
 		_create: function() {
 			equal( this._classes( "test" ), "test class1 class2" );
 			equal( this._classes( "test2" ), "test2 class3" );
+			equal( this._classes( "testEmpty" ), "testEmpty",
+				"Computed value of empty-string-valued class key has no extra spaces" );
 			equal( this._classes( "test test2" ), "test2 class3 test class1 class2" );
 		}
 	});

ui/widget.js

@@ -410,7 +410,11 @@ $.Widget.prototype = {
 
 		while ( i-- ) {
 			out.push( parts[ i ] );
-			out.push( classes[ parts[ i ] ] );
+
+			// Avoid extra spaces in the return value resulting from pushing an empty classes value
+			if ( classes[ parts[ i ] ] ) {
+				out.push( classes[ parts[ i ] ] );
+			}
 		}
 
 		return out.join( " " );