Setting maxLength to undefined yields weird result on Phantom

Apparently it sets maxLength to "524288":
https://travis-ci.org/madrobby/zepto/builds/29254260

Use a custom property to avoid magic browser behavior around certain
properties.

test/zepto.html

@@ -1742,14 +1742,14 @@ <h1>Zepto Core unit tests</h1>
       },
 
       testPropSetterErase: function(t){
-        var input = $('<input readonly maxlength=5>')
+        var input = $('<input readonly>')
         t.assertIdentical(input, input.prop('readonly', false))
         t.assertFalse(input.prop('readonly'))
 
-        t.assertIdentical(5, input.get(0).maxLength)
-        t.assertIdentical(input, input.prop('maxlength', undefined))
-        t.assertIdentical(0, input.get(0).maxLength)
-        t.assertIdentical(0, input.prop('maxlength'))
+        input.get(0)._foo = 'bar'
+        t.assertIdentical(input, input.prop('_foo', undefined))
+        t.assertUndefined(input.get(0)._foo, 'custom property should be cleared')
+        t.assertUndefined(input.prop('_foo'), 'prop should reflect cleared property')
       },
 
       testAttrNoElement: function(t){