we can't pass Encoding instance to String#encode

however it is possible to pass string representation of that instance.
only use encodings which have a converter present

this is required to _run_ the test.
add new line to end of file
UTF-8 can represent any string, convert then test

- Our Regexp is in UTF-8
- No matter what string is passed to method we can represent it with UTF-8

So let's first convert given argumentinto UTF-8 then test it with our Regexp.