library/form: Fix return value of `getName()' not being filterd the first time it's called

`Form::getName()' auto-detects the form's name if it's not set and sets the form's name using `Zend_Form::setName()'.
`Zend_Form::setName()' then filters the form name stripping backslash '\' characters. The first call to `Form::getName()'
did not regard the filtering and thus returned the unfiltered name which led to `Form::wasSent()' returning `false'.

refs #5525

library/Icinga/Web/Form.php

@@ -585,8 +585,8 @@ class Form extends Zend_Form
         if (! $name) {
             $name = get_class($this);
             $this->setName($name);
+            $name = parent::getName();
         }
-
         return $name;
     }