Prefer forward conversion if registered

[eischaefer]

When a forward conversion is registered to getFreezeConversionMap(), that explicit conversion is now preferred over the (implicit) inverse of a (backward) conversion registered to getFreezeConversionInverseMap(). This behavior is consistent with the preexisting preference for explicit forward conversion of certain simple types (_freeze_types_plain) and non-iterables and strings (i.e., excluded by isIterableNotString()).

The modified behavior can be seen in the example below:

import frozendict
import types

dic = {"a": "A"}
prox = types.MappingProxyType(dic)

frozendict.register(types.MappingProxyType, frozendict.frozendict)

frozen = frozendict.deepfreeze(prox)
isinstance(frozen, frozendict.frozendict)  # Now True, was False
isinstance(frozen, types.MappingProxyType)  # Was True, now False

As a practical use case, I'm currently working on a project in which I'd like to use deepfreeze() to ensure that a nested mappingproxy is hashable.

[Marco-Sulla]

Uh, a subtle bug, good.
Can I ask you the favor to add a little unit test for this case under test/test_freeze.py?

[Marco-Sulla]

I thought you add a test with MappingProxy, instead you preferred a more robust solution.