Fix issue with tail content-range headers

[BennyDeBock]

Adds testing for the different content-range options (start, within and tail)

Fix the bytecount calculation of the content-range headers

Add tests that should fail.

closes #2836

[0xTim]

(Adding review so GH reminds me when it's updated) - this looks great but we need to test edge cases more and ensure videos still play back etc if you could do a manual test as well)

[0xTim]

Thanks!

[0xTim]

Will roll this is with a bunch of other PRs today