Fix compile in Scala 2.10.1+

Scala 2.10.1 includes a type checking bug fix that causes compilation to
fail.  Add casts to work around this (effectively restoring the previous
behavior).
Merge pull request #1509 from lift/pmb_liftscreenfix

Fix compile in Scala 2.10.1+

Scala 2.10.1 included a type checking bug fix that caused compilation to
fail for AFilter pattern matches. Add casts to work around this (effectively
restoring the previous behavior of scalac, but explicitly).