Effects: set default easing using jQuery.easing._default

Fixes gh-2219
Close gh-2218

src/effects.js

@@ -289,7 +289,10 @@ function Animation( elem, properties, options ) {
 		animation = deferred.promise({
 			elem: elem,
 			props: jQuery.extend( {}, properties ),
-			opts: jQuery.extend( true, { specialEasing: {} }, options ),
+			opts: jQuery.extend( true, {
+				specialEasing: {},
+				easing: jQuery.easing._default
+			}, options ),
 			originalProperties: properties,
 			originalOptions: options,
 			startTime: fxNow || createFxNow(),

src/effects/Tween.js

@@ -13,7 +13,7 @@ Tween.prototype = {
 	init: function( elem, options, prop, end, easing, unit ) {
 		this.elem = elem;
 		this.prop = prop;
-		this.easing = easing || "swing";
+		this.easing = easing || jQuery.easing._default;
 		this.options = options;
 		this.start = this.now = this.cur();
 		this.end = end;
@@ -105,7 +105,8 @@ jQuery.easing = {
 	},
 	swing: function( p ) {
 		return 0.5 - Math.cos( p * Math.PI ) / 2;
-	}
+	},
+	_default: "swing"
 };
 
 jQuery.fx = Tween.prototype.init;