[887] Respect the form buffer!

resolves #887

All it took:
```ruby
# This method calls the block it's passed (in our case, the `f.inputs` block)
# and wraps the resulting HTML in a fieldset. If your block happens to return
# nil (but it otherwise built the form correctly), the below override passes
# the most recent part of the Active Admin form buffer.
def field_set_and_list_wrapping(*args, &block)
  block_given? ? super{ yield || form_buffers.last } : super
end
```