Remove format 'html' in Rest controller

In fact, 'html' is not supported.

And Rest controller returns string 'Array' if array is passed on 1.8/develop.

On 1.7/master an error occurs:
```
Fuel\Core\PhpErrorException [ Warning ]:
strpos() expects parameter 1 to be string, array given
DOCROOT/index.php @ line 78
```