Fix possible null reference

* 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
Add regression test for #13.
Merge branch 'checkboxproblem' of https://github.com/mlitbk/robobrowser
Combine form-related fixtures.
Bump version to 0.3.1
Resolve conflicts.