Explicitly call IO::File in Dancer::FileUtils

For perls 5.14+ it should be a non-op, but it can
make a difference for older perls. More specifically
for Dancer, it can make a difference for the call
to 'autoflush' in Dancer::Logger::File
(see dagolden/Dancer-Session-Cookie#9)

Fixes #954
don't go boom is can't autoflush

looks like older perls don't auto-convert to IO::File
and can't autoflush
see https://github.com/PerlDancer/Dancer/issues/954
changelog
Merge branch 'io-file' into devel