Fixes zero padding calculation

[colinta]

Per our discussion on Slack. Includes a test that was failing (now passing).

To hunt this down I basically started implementing SHA1 on my own, and then lined it up with your code (which, btw, is way more straightforward than other implementations out there, esp. JS ones). I couldn't figure out the zero padding calculation: 64 - modBy 64 (List.length bytes + 1 + 8), so I just replaced it with the formula I had ended up with: modBy 64 (56 - modBy 64 (byteCount + 1)) and all the tests passed! More verbosely, this calculation is:

bytesLen =
    byteCount + 1

lastChunkLen =
    modBy 64 bytesLen

zeroPadLen =
    modBy 64 (56 - lastLen)

elm-test

elm-test 0.19.0-rev6
--------------------

Running 36 tests. To reproduce these results, run: elm-test --fuzz 100 --seed 67610542048265 /Users/colinta/Code/elm/elm-sha1/tests/Tests.elm


TEST RUN PASSED

Duration: 1761 ms
Passed:   36
Failed:   0

[TSFoster]

Thank you so much for tracking this down and fixing it while I was asleep (I'm in the UK)! I will get this published ASAP with a note in the README about the issue, and I'll create a PR for romariolopezc/elm-hmac-sha1 bumping the version of this library.