Fix comparison between omdict and dict.

Comparing an `omdict` with a `dict` always returns `False` in Python 3, since `__eq__` checks for an attribute named `iteritems`, which is renamed to `items`. This patch checks for the appropriate method depending on Python version, uses `six.iteritems` for iteration, and restores an existing test to prevent regressions. Needed for https://github.com/gruns/furl/pull/35.