Don't overwrite mr._request with None in FieldSet.bind

[benbeanfield]

The FieldSet constructor accepts a request kwarg. If it is supplied, then self._request will be non-None.

FieldSet.bind() also accepts a request object as a kwarg with a default of None. If a request is not supplied to bind(), then bind() will overwrite mr._request with None:

    def bind(self, model=None, session=None, data=None, request=None,
             with_prefix=True):
        # ...
        mr = object.__new__(self.__class__)
        mr.__dict__ = dict(self.__dict__)
        # ...
        mr._request = request
        return mr

bind() should not set mr._request if request is None or falsy.