Fix bug with passing wrong args to __init__ in Blob.__setstate__.

`__getstate__` returns `self.data(), self._filler`, where `self.data()` may be `None`.  But when `__setstate__` initializes the `Blob` and `state[0]` is `None`, it will wrongly pass the filler (`state[1]`) as the second positional argument to `__init__`.  But that is `dtype`, not `filler`.  Using a keyword argument fixes this.