IdpAttribute#getValues()

[Rod Widdowson]

I finally got bored with the "deprecated class" warning in IdPAttribute and
spent some time looking at it.

It turns out that the code in IdPAttribute is old enough to below to the
days when we were still doing the V2 type

attribute.getValues().add(thing).

And in order to enforce the non-nullability in the face of that the code is
using some now deprecated classes in guava.  

#getValues() is the only @Modifiable set returned from IdPAttribute; all the
others are @Unmodifiable and do the ImmutableSet.copyOf() thing.  

I have prepared a checkin to fix this.  I'd estimate that more than 95% of
the code changes were to tests where the getSet().add() paradigm was a handy
shortcut.

Before I check this in I wanted to check that no one out there thinks that
this might be a retrograde step.

Rod