fix test failures on big-endian systems

This fixes test failures on big-endian systems.  You can see failing test output here:

  https://launchpadlibrarian.net/134964454/buildlog_ubuntu-raring-powerpc.haskell-cipher-aes_0.1.7-1_FAILEDTOBUILD.txt.gz

I've tested this on the current Ubuntu development branch, on both i386 and powerpc.