Pop the first item for a list on the dict.  Afterwards the
        key is removed from the dict, so additional values are discarded:

        >>> d = MultiDict({"foo": [1, 2, 3]})
        >>> d.pop("foo")
        1
        >>> "foo" in d
        False

        :param key: the key to pop.
        :param default: if provided the value to return if the key was
                        not in the dictionary.

def pop(self, key, default=_missing):
        """Pop the first item for a list on the dict.  Afterwards the
        key is removed from the dict, so additional values are discarded:

        >>> d = MultiDict({"foo": [1, 2, 3]})
        >>> d.pop("foo")
        1
        >>> "foo" in d
        False

        :param key: the key to pop.
        :param default: if provided the value to return if the key was
                        not in the dictionary.
        """
        try:
            return dict.pop(self, key)[0]
        except KeyError, e:
            if default is not _missing:
                return default
            raise BadRequestKeyError(str(e))