Improve the old encoding tests

make them a bit more sensible and expect them to pass
Show which vim is being used
Accept (and return) character strings

closes gh-1.

Thanks, RJBS!
Test character string handling
Pretend "unencoded" is a word
v0.23

  - Attempt to do the right thing with character strings:
    Encode them in UTF-8, tell vim the file encoding (UTF-8),
    and return a (decoded) character string.
    Thanks to Ricardo Signes for the very helpful report (gh-1).