Normalize bh & rails fields_for arguments
* All old tests passes, so should be backward compatible
* Closes #21

See https://github.com/rails/rails/blob/7a085dac2a2820856cbe6c2ca8c69779ac766a97/actionview/lib/action_view/helpers/form_helper.rb#L1509
Merge branch 'pr/22'
Add missing test for fields_for with record object

See 7f2586b
Merge branch 'fix-fields-for'