encoding-guesser: Don't guess UTF-8 for ASCII if it is the fallback.

When the text presented to the encoding guesser is all ASCII, normally
the encoding guesser will report ASCII as its guess.  But if the
fallback encoding is UTF-8, then it reports UTF-8 instead.
Unfortunately, this makes the encoding guesser a bit harder to test,
because guesses depend on the system's locale.  It's easier to test
if all-ASCII always yields ASCII as the guess, so this changes the
encoding guesser to do that.

This fixes a test failure on Mac OS X.  Thanks to Jeremy Lavergne for
reporting the problem.