Added a test that fails when the friendlyName of the requested attribute
is not the same with the name of the internal attribute (even though the
OIDs and the internal representation names of the attribute are the same)

Instead on relying on the FriendlyName from metadatata, use the name
of the internal representation of an attribute in order to perform
filtering. Resolves IdentityPython#422

As explained in
IdentityPython#423 (comment) , ava
cannot contain an 'surName' key, it should be named 'sn'

Since acs can't be None ( it get's a value in __init__()
https://github.com/rohe/pysaml2/blob/master/src/saml2/assertion.py#L319)
there is no reason to test for it. So we add a default value to acs
using ac_factory() before passing it to filter_on_attributes