Expand unit tests
Expand unit tests

- Add test for double replacement with a reversed order since the order
  within the dictionary actually matters.
- Remove upper-case umlaut replacements from standard test since the
  string replacement happens after the input text is converted to
  lower-case, so these string replacements are not used.
- Add additional tests for umlaut replacements without a conversion to
  lower-case to ensure that the replacement of upper-case characters is
  independent of the lower-case characters.

ctolkien/Slugify,ctolkien/Slugify