Stop falling back to reading properties in fn.attr()

That's what `prop()` is for. This breaking change is for jQuery 1.6
compatibility. Also, missing properties now return `undefined` instead
of `null`.

Fixes #1080

src/zepto.js

@@ -633,9 +633,7 @@ var Zepto = (function() {
  attr: function(name, value){
  var result
  return (typeof name == 'string' && !(1 in arguments)) ?
- (!this.length || this[0].nodeType !== 1 ? undefined :
- (!(result = this[0].getAttribute(name)) && name in this[0]) ? this[0][name] : result
- ) :
+ (0 in this && this[0].nodeType == 1 && (result = this[0].getAttribute(name)) != null ? result : undefined) :
  this.each(function(idx){
  if (this.nodeType !== 1) return
  if (isObject(name)) for (key in name) setAttribute(this, key, name[key])

test/zepto.html

@@ -1798,7 +1798,7 @@ <h1>Zepto Core unit tests</h1>
  t.assertEqual('someOtherName', els.attr("data-name"))
  t.assertEqual('someOtherId', $('#attr_2').attr('data-id'))
 
- t.assertNull(els.attr("nonExistentAttribute"))
+ t.assertUndefined(els.attr("nonExistentAttribute"))
 
  els.attr("data-id", false)
  t.assertEqual("false", els.attr("data-id"))
@@ -1822,7 +1822,7 @@ <h1>Zepto Core unit tests</h1>
  var el = $('<div data-name="foo">')
  t.assertIdentical(el, el.attr('data-name', undefined), 'setter should return self')
  t.assertNull(el.get(0).getAttribute('data-name'), 'attribute should be erased')
- t.assertNull(el.attr('data-name'), 'attr should reflect erased attribute')
+ t.assertUndefined(el.attr('data-name'), 'attr should reflect erased attribute')
  },
 
  testProp: function(t){
@@ -1909,18 +1909,18 @@ <h1>Zepto Core unit tests</h1>
  testAttrNullUnset: function(t){
  var el = $('<div id=hi>')
  el.attr('id', null)
- t.assertIdentical('', el.attr('id'))
+ t.assertUndefined(el.attr('id'))
 
  el.attr('id', 'hello')
  el.attr({ id:null })
- t.assertIdentical('', el.attr('id'))
+ t.assertUndefined(el.attr('id'))
  },
 
  testRemoveAttr: function(t) {
  var el = $('#attr_remove')
  t.assertEqual('boom', el.attr('data-name'))
  el.removeAttr('data-name')
- t.assertNull(el.attr('data-name'))
+ t.assertUndefined(el.attr('data-name'))
  },
 
  testRemoveMultipleAttr: function(t) {
@@ -1929,8 +1929,8 @@ <h1>Zepto Core unit tests</h1>
  t.assertEqual('someName1', el.attr('data-name'))
 
  el.removeAttr('data-id data-name')
- t.assertNull(el.attr('data-id'))
- t.assertNull(el.attr('data-name'))
+ t.assertUndefined(el.attr('data-id'))
+ t.assertUndefined(el.attr('data-name'))
  },
 
  testRemoveAttrNoElement: function(t){
@@ -1939,9 +1939,9 @@ <h1>Zepto Core unit tests</h1>
 
  var els = $('<b rel=up></b> <i rel=next></i>')
  t.assertIdentical(els, els.removeAttr('rel'))
- t.assertNull(els.eq(0).attr('rel'))
+ t.assertUndefined(els.eq(0).attr('rel'))
  t.assertUndefined(els.eq(1).attr('rel'))
- t.assertNull(els.eq(2).attr('rel'))
+ t.assertUndefined(els.eq(2).attr('rel'))
  },
 
  testData: function(t) {
@@ -2615,8 +2615,9 @@ <h1>Zepto Core unit tests</h1>
  },
 
  testBoolAttr: function (t) {
- t.assertEqual($('#BooleanInput').attr('required'), true)
- t.assertEqual($('#BooleanInput').attr('non_existant_attr'), undefined)
+ var el = $('#BooleanInput')
+ t.assertEqual('', el.attr('required'))
+ t.assertUndefined(el.attr('non_existant_attr'))
  },
 
  testDocumentReady: function (t) {