Crash when using min/max and passing strings to Decimal fields

[jackqu7]

If you have min/max on a Decimal field, and you pass the value as a string (which is supposed to be supported,) a crash occurs.

    field.marshal(mapper_session)
kim/field.py:344: in marshal
    run_pipeline(self.marshal_pipes, session, self, **opts)
kim/pipelines/base.py:176: in run_pipeline
    pipe_func(session)
kim/pipelines/base.py:104: in inner
    return pipe_func(session)
kim/pipelines/numeric.py:46: in bounds_check
    if min_ is not None and session.data < min_:```