Always returning a short character in short_from

The function short_from(name) give a character to use as a short option form.
There was a bug is all the characters were already used: the function would
then return the full name.

So for instance, in
    options = Parser.new do |p|
      p.option :a, "a"
      p.option :b, "b"
      p.option :ab, "ab"
    end.process!
short_from("ab") would return "ab", which mean that the call to OptParse would
look like
    p.on("-ab", "--ab", "ab")...
This create an option --ab whose short form is -a and which needs a
mandatory argument b
(and passing --help give something like
-a --abb ab
because we ask for a mandatory argument b)

This commit changes short_from to return the first letter if all are taken.
In this case, the call to OptParse is
    p.on("-a", "--ab", "ab")...
which means that the previous short form -a for --a is overrided but has no
other ill-effects.

Note: in opt parse, the last defined short form win, except that p.on have
more priority. In particular, if both '-v' and '-V' short form are taken,
then the call to
      short = @used_short.include?("v") ? "-V" : "-v"
      p.on_tail(short, "--version", "Print version")
in microoptparse will not override these switch for the --version option
which is what we want.
Merge pull request #7 from DamienRobert/shortoptions

Always returning a short character in short_from