correct jqmData behavior to match our tests

Author: johnbender

js/jquery.mobile.core.js

@@ -260,7 +260,14 @@ define( [ "jquery", "../external/requirejs/text!../version.txt", "./jquery.mobil
 			if ( prop ) {
 				prop = $.mobile.nsNormalize( prop );
 			}
-			result = this.data.apply( this, arguments.length < 2 ? [ prop ] : [ prop, value ] );
+
+			// undefined is permitted as an explicit input for the second param
+			// in this case it returns the value and does not set it to undefined
+			if( arguments.length < 2 || value === undefined ){
+				result = this.data( prop );
+			} else {
+				result = this.data( prop, value );
+			}
 		}
 		return result;
 	};