Fix possible AttributeError

I came across this problem when I called get_forms() on a page with a form that contains checkboxes. I got this error:
AttributeError: "'NoneType' object has no attribute 'lower'"
on this line:
while tags and tags[0].get('name').lower() == name:
One of the inputs didn't have a 'name' attributed and it caused this error.
I added a check to ensure that get('name') does return a valid object before calling lower() on it, and also a test case for this problem.

* Fix AttributeError that occures when searching for inputs with the same
name, and of the inputs doesn't have 'name' attribute. We need to check
if the object exists before calling lower().

* Add mock response and test for form where there are checkboxes with the
same name and one input with no name