Asking the parent class for its field list if ours is not present.

Previously, if you inherited from a ticket sharing base class you did
not inherit your parent's field list. Now, we check our fields list,
and if it is empty, we use the parent's field list.

This propagation will continue all the way up to the
TicketSharing::Base class.
Merge pull request #8 from zendesk/josh/ask_for_your_parents_field_list

Asking the parent class for its field list if ours is not present.