Allow children of Zend_Form to handle setDefaults to array conversion by themselves

I suggest removing of the `array` typehint in the `Zend_Form::setDefaults()` method as it prevents us from overriding it to accept `Zend_Db_Row`, convert it to array and call the `setDefaults` from parent.

```
Declaration of Form::setDefaults() should be compatible with Zend_Form::setDefaults(array $defaults)
```

I can also improve it further and add an automatic handling of passed objects with toArray() method, what do you think?