Perform UTF8-decoding on the buffer provided by the 'tar' library before parsing it's contents.

Some packages (like Octree 0.5) use Unicode characters in their Cabal file that
require more than one byte in UTF8 representation. The tar library doesn't care
about UTF8, though, it simply passes a string of Word8 bytes. This means that
hackage-db has to perform UTF8 decoding before parsing the buffer with Cabal to
avoid syntax errors.

Thanks to Roman Beslik for finding this issue.