Custom mapping fails for non-latin keys

[aziznatour]

The following code should work, but does not:

getSlug("ديسكورد", {
   custom: {
      "ديسكورد": "discord"
   }
)};

Result: dyskwrd
Expected result: discord

[kalligator]

Seems to be a persistent issue if the key is more than one non-latin characters, not just for custom mappings.

[kalligator]

I have fixed it as far as the custom mappings go, see PR #132 above.