A few behavior changes

[cag]

I apologize for the title, but I realize this work is actually about multiple things.

1. to_native and to_primitive of StringType should behave the same. When I serialize objects with a string field through json.dumps, I don't want to have special logic handling to_primitive when the field passed to the model is not a string. Yes, the database should be migrated, but oftentimes it is nice to be able to cast existing objects into strings, and I do expect to_primitive to give me back a str on StringTypes by default.
2. Interoperability: my local python distro makes .eggs and does not ship with mapping_tests.py
3. There is an undocumented (AFAIK) assumption by Python in which exceptions are expected to be hashable in order to be added to a stack trace seen_excs set. Since FieldErrors override __eq__, Python states that they are subsequently not hashable unless they also implement __hash__, so I have done so.

A side effect of (1) is that StringTypes also must handle None for to_primitive, as serializing objects with ListTypes and DictTypes containing StringTypes throw ConversionErrors when they contain None.

[cag]

I still don't have mapping_tests.py, but at least this checks out according to Travis...

[lkraider]

The assumption is that to_primitive will only operate on to_native converted data. Any other use is not supported - ie: if you are calling to_primitive outside of Model.serialize you need to take care of the data state yourself.

The problem with your __hash__ implementation is that the FieldError.messages attribute is mutable, that means its hash can change during its lifetime. A correct fix would be to make it immutable.

[lkraider]

Regarding point 3, see also discussion in #452 .

[lkraider]

Work is being done to add immutability to the exceptions, but I am rejecting this PR as it adds incorrect handling for the hashing.

Also, the to_primitive == to_native use case added here is not supported.

Please see PR #477 if you want to help with the immutability discussion.