[PERF] Don't Attempt To Sanitive Unsanitizeable

This adds an identity check for falsy values before the value is
sanitized. Currently we do work to sanitize things like `undefined`.

Author: chadhietala

packages/morph-attr/lib/main.js

@@ -110,7 +110,7 @@ AttrMorph.prototype.setContent = function (value) {
   if (this.lastValue === value) { return; }
   this.lastValue = value;
 
-  if (this.escaped) {
+  if (this.escaped && value) {
     var sanitized = sanitizeAttributeValue(this.domHelper, this.element, this.attrName, value);
     this._update(sanitized, this.namespace);
   } else {

packages/morph-attr/tests/attr-morph-test.js

@@ -17,6 +17,20 @@ test("can update a dom node", function(){
   equal(element.getAttribute('id'), 'twang', 'id attribute is set');
 });
 
+test("setting content to undefined calls _update with undefiend", function(){
+  var element = domHelper.createElement('div');
+  var morph = domHelper.createAttrMorph(element, 'id');
+  var update = morph._setContent;
+  var calledWith;
+  morph.setContent(undefined);
+  morph._update = function(value) {
+    calledWith = value;
+    return update.apply(morph, arguments);
+  };
+  ok(calledWith === undefined);
+  equal(element.id, '');
+});
+
 test("can clear", function(){
   expect(0);
   var element = domHelper.createElement('div');