Inconsistent normalization mapping of German umlauts

Hey guys,

String.normalize/1 behaves inconsistently when normalizing German umlauts to ASCII:

    ä -> a
    ö -> o
    ü -> ue

The same holds true for the uppercase equivalents of those characters. Hoping to speak on behalf of all German MooTools developers, I suggest the following mapping:

    ä -> a
    ö -> o
    ü -> u

I would be very happy, if this little fix would make into `mootools-more`.

Best regards,
Martin