0.9.6.4, defaultIndexGenerator doesn't properly show file if its name contains Russian letters

[alogic0]

Detected on Debian, Ubuntu, locale en_US.UTF-8. More precisely, it fails on characters in range \1024 - \1151, because T.decodeUtf8 thinks it is correct UTF-8 sequence. Problem is here
https://github.com/snapframework/snap-core/blob/0.9-stable/src/Snap/Util/FileServe.hs#L355
Example:

cpp -E FileServe.hs > tmp.hs
ghci tmp.hs
ghci> T.decodeUtf8 $ S.pack "йфыяхъ"
"9DKOEJ"

It must be exeption, but it isn't like in the case of Latin symbols:

ghci> T.decodeUtf8 $ S.pack "üéî"
"*** Exception: Cannot decode byte '\xfc' ...

So we need just do T.pack on such strings.