Support standard Python semantics for proxied java objects that
implement java.util.Map. Such maps may or may not be concurrent.

Note that d[key_not_present] no longer returns None, but raises
KeyError, which means this behavior now matches standard dict
semantics.

Fixes http://bugs.jython.org/issue1631
Skip a couple of dict tests not relevant to proxied java.util.Map objects.
Still need to implement comparisons for such proxied objects other than __eq__.