fix #939, .attr('value') should differs from .val() on <input>

With the patch, **Zepto** gets more consistent with **jQuery**.
There should be no detecting on `<input>` in method `.attr()`.
> `(name == 'value' && this[0].nodeName == 'INPUT') ? this.val() :`

- `.val()` does not influence `.attr('value')`
- however, once `.attr('value')` changes, `.val()` changes too

Priority: `.attr('value')` > `.val()`