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

Fix #939. 

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~~

- usually, `.attr('value')` > `.val()` - .attr('value', v) would change both
- however, once after applying `.val(v)`, `.attr('value')` and `.val()` separates to manage their own

~~Priority: `.attr('value')` > `.val()`~~