Fix corner case when all chars are uppercase or lowercase

src/snake_case.nim

@@ -16,6 +16,7 @@ proc toSnakeCase*(str: string): string =
       elif ch in {'-', '_', '.', '+'}:
         if result.len > 0 and result[^1] != '_':
           result.add '_'
+      prv = ch
 
 proc fromSnakeCase*(str: string): string =
   ## Convert from SNAKE_CASE to camelCase