Multimock fails for spec'ed multimethods

[bsima]

If a multimethod is spec'ed tih spec/fdef, then you get an error like:

java.lang.ClassCastException: clojure.spec.test.alpha$spec_checking_fn$fn__1352 cannot be cast to clojure.lang.MultiFn

The fix is probably to check if the multimethod we are mocking is spec'ed, and if so then figure out how to get the actual function instead of the spec_checking_fn