fix weighting with Digest class

[rcombs]

Previously, the Digest class didn't support passing weights to the push routine. There doesn't appear to have been any reason for this.

[welch]

good catch! would you add a test or two for the change?

[rcombs]

There don't seem to be any tests for the weight arg at all, so maybe it'd be best to start there? Not sure exactly what the best tests for this would look like, though.