Position: Make sure of is treated as a CSS selector [SO-2921446]

Author: Rômulo Göelzer Portolann Malkiewiez

ui/position.js

@@ -141,7 +141,10 @@ $.fn.position = function( options ) {
 	options = $.extend( {}, options );
 
 	var atOffset, targetWidth, targetHeight, targetOffset, basePosition, dimensions,
-		target = $( options.of ),
+		target = typeof options.of === "string" ?
+				$( document ).find( options.of ) :
+				$( options.of ),
+
 		within = $.position.getWithinInfo( options.within ),
 		scrollInfo = $.position.getScrollInfo( within ),
 		collision = ( options.collision || "flip" ).split( " " ),