Add exclude_extra_attributes_from_session option to RackCAS configuration

[ccadden]

Our CAS implementation recently changed to have a very large array come back in extra_attributes. As a result we were accidentally exceeding the maximum allowed cookie size in one of our applications by putting the full list in the session store.

Our app doesn't need any of the extra attributes. As a work around I had to use the whitelist option with a dummy attribute value to get it to throw out all the unneeded data.

config.rack_cas.extra_attributes_filter = %w(dummy_value)

I'm hoping this has values to others who want a straightforward way to throw out the extra attributes.