Critical problems with auth db connections

[dbnski]

This pull request resolves two problems:

1. RT::Handle forces utf8 encoding on the RT database connection, so ExternalAuth::DBI MUST do the same since it does not perform on-the-fly character set conversion. Otherwise RT database may throw errors when non-utf8 characters are present in queries (e.g. accented characters in RealName field), or we may end up writing garbled text into RT tables.
2. New RT introduced DatabaseExtraDSN, but the module currently does not respect it.