return null instead of method on non-object error
Fixed the submission of form fields with same name and without id

Replaces #26 which was not working properly
Refs Behat/Mink#410 for new tests
Merge pull request #37 from stof/fix_form_field_without_id

Fixed the submission of form fields with same name and without id
Merge pull request #24 from arcturial/master

GetCurrentUrl() return null instead of fatal error