Generate value() commands instead of key() commands on elements

I was getting things like:
```php
$this->byId("name")->keys("test");
```
from the formatter, which was giving me the following error when running the test:
`BadMethodCallException: The command 'keys' is not existent or not supported yet.`

Based on the file at:
https://github.com/giorgiosironi/phpunit-selenium/blob/master/PHPUnit/Extensions/Selenium2TestCase/ElementCommand/Value.php
and that it says that the ElementCommand_Value class extends the SessionCommand_Keys class, I think (hope) that my change will do the right thing.