FlexEntity.flatten() will convert unknown types to str

[six8]

It should unknown types intact. Converting to str could be an encoding issue for unicode-like values. Values that are already Python types such as UUID should stay a UUID.

Instead, fields.get_field_for_type(val) should be used instead to guess the best type for the value.

https://github.com/six8/springfield/blame/develop/src/springfield/entity.py#L356

        elif not isinstance(val, (basestring, int, float, long)):
            val = str(val)