zero fill to make abstract-chunk-store compliant

[ghost]

I think zeroPadding: false violates the expectations of the abstract-chunk-store test suite: https://github.com/mafintosh/abstract-chunk-store/blob/master/tests/index.js#L25-L34

With zeroPadding: false, chunks are written to the store that are smaller than the chunkLength, which according to the abstract-chunk-store tests should result in an error.