Take next character into account for *uppercase* german umlauts

[ThomasLandauer]

According to https://github.com/cocur/slugify/blob/master/Resources/rules/austrian.json#L3 and https://github.com/cocur/slugify/blob/master/Resources/rules/german.json#L3 "Österreich" will be slugified to "OEsterreich", but it should be "Oesterreich". However, when it's all uppercase "OESTERREICH" is correct (not "OeSTERREICH"). So the perfect (?) solution probably would be to check if the next character is uppercase too, then go for E or e.