Core: Stop trimming value inside required method

There was never much of a point anyway, and it causes problems.

Fixes #1498

src/core.js

@@ -1143,7 +1143,7 @@ $.extend( $.validator, {
 			if ( this.checkable( element ) ) {
 				return this.getLength( value, element ) > 0;
 			}
-			return $.trim( value ).length > 0;
+			return value.length > 0;
 		},
 
 		// http://jqueryvalidation.org/email-method/