Allow uploading docs as tar.gz

[phadej]

When making cabal haddock --haddock-for-hackage, the .tar.gz package is produced.

If I upload it manually to Hackage,

I get For this resource the content-type was expected to be application/x-tar, rather than application/gzip

which is quite inconvenient

[gbaz]

I agree, would love a patch for this.