Adding attributes to fieldsets. Legends are optional for fieldsets.

As the comment says: Every collection is wrapped by a fieldset if needed

A legend is not required for a fieldset.
* Now, if a label is specified, a legend will be created.
* Before, a fieldset would not be wrapped around the markup unless a label was set.
* Now, a fieldset will be wrapped around the markup if $this->shouldWrap is true, regardless if a label has been set for the legend.