Update length if binary property is stream

In the Filesystem implementation, the `length` was being set to the value of the property, which was a Stream. This PR sets stats the stream and sets the length based on that.

I'm not sure how this works in Doctrine DBAL / Jackalope, but I guess the `->length` property should always be an integer.
hmm. in my case the value of the property was the stream resource. Should I not be returning the stream resource as the value of the node?