[fix][#31697] Request params not removed/updated

This fixes:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31697

Additionally I added some methods commonly used by extension developers:

In `JForm` :
```PHP
public function getAttribute($name, $default = null)
public function getData()
public function getXml()
```

In `JFormField`:
```PHP
public function getAttribute($name, $default = null)
```