form_group doesn't require name when passing params

More often than not, I'd like to attach a class to a `form_group` but it always forces you to specify a name. So before:
```ruby
f.form_group :nil, :class => 'my-class' do
  # ... stuff
end
```
Now:
```ruby
f.form_group :class => 'my-class' do
  # ... stuff
end
```

Great form builder by the way. Really like it.