zero fill to make abstract-chunk-store compliant

I think [`zeroPadding: false`](https://github.com/feross/chunk-store-stream/blob/c999d61a23c2b74f8fd7b810358a8c128ee85899/write.js#L23) 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.