Provide a way to request no portals

[pilaf]

Currently FmRest::Spyke::Relation doesn't provide an easy way to specify you want no portals.

Some ideas for the API for this:

Model.portal(nil) # could work as the first call, but multiple calls to `.portal` currently add to the list of portals... how to deal with that?

Model.portal([]) # ugly?
Model.clear_portals # also ugly?

Note: the Data API needs portal=“[]” for this to work.

[pilaf]

3a6af3d