Handle non-ASCII keys and values properly

- Ask if the key is UTF-8 instead of guessing, and convert as needed
- Just use SvPVutf8() instead of trying to detect the encoding
  (this way, downgraded character strings that look like UTF-8 can be inserted)
Remove unused isUTF8 function
Merge pull request #67 from ilmari/utf8-fixes

UTF-8 fixes