Second argument to `attr/prop/data` is always a setter

This enables setting `undefined` value
Improve tests that verify attr/prop/data setter erasing
Shorten checking if 2nd argument was passed

Continues on 761201e80fa81750d3cefe3d4d3cd2801b33362b
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.
Merge branch 'attr-setter'

Closes #952, fixes #942