Position: Make sure of is treated as a CSS selector

Closes gh-1955

tests/unit/position/core.js

@@ -113,7 +113,9 @@ QUnit.test( "positions", function( assert ) {
 } );
 
 QUnit.test( "of", function( assert ) {
-	assert.expect( 9 + ( scrollTopSupport() ? 1 : 0 ) );
+	assert.expect( 10 + ( scrollTopSupport() ? 1 : 0 ) );
+
+	var done = assert.async();
 
 	var event;
 
@@ -225,6 +227,21 @@ QUnit.test( "of", function( assert ) {
 		top: 600,
 		left: 400
 	}, "event - left top, right bottom" );
+
+	try {
+		$( "#elx" ).position( {
+			my: "left top",
+			at: "right bottom",
+			of: "<img onerror='window.globalOf=true' src='/404' />",
+			collision: "none"
+		} );
+	} catch ( e ) {}
+
+	setTimeout( function() {
+		assert.equal( window.globalOf, undefined, "of treated as a selector" );
+		delete window.globalOf;
+		done();
+	}, 500 );
 } );
 
 QUnit.test( "offsets", function( assert ) {
@@ -334,6 +351,7 @@ QUnit.test( "using", function( assert ) {
 			assert.deepEqual( position, expectedPosition, "correct position for call #" + count );
 			assert.deepEqual( feedback.element.element[ 0 ], elems[ count ] );
 			delete feedback.element.element;
+			delete feedback.target.element.prevObject;
 			assert.deepEqual( feedback, expectedFeedback );
 			count++;
 		}

ui/position.js

@@ -148,7 +148,12 @@ $.fn.position = function( options ) {
 	options = $.extend( {}, options );
 
 	var atOffset, targetWidth, targetHeight, targetOffset, basePosition, dimensions,
-		target = $( options.of ),
+
+		// Make sure string options are treated as CSS selectors
+		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( " " ),