lxml.html.formfill: Fix textarea form filling.

**Problem**
When a textarea is filled using `lxml.html.formfill`, the `name` attribute (and all other attributes) are removed because `input.clear()` is called before the textarea is filled.

**Effect**
The textarea field is no longer usable and the field and its value does not show up when submitting the form (e.g. with `lxml.html.submit_form`).

**Cause**
The `input.clear()` resets the element and removes all attributes, as described in the docstring:
```
input.clear():
Resets an element.  This function removes all subelements, clears
all attributes and sets the text and tail properties to None.
```

**Fix**
I've removed the `input.clear()`. I'm not sure why it was there in the first place and couldn't find out.
I've added a regression test.