Convert user/pass tokens to ASCII in db.rb

This commit fixes an Encoding::CompatibilityError incompatible
encoding regexp match (ASCII-8BIT regexp with UTF-8 string) when
sanitizing non-printable tokens from a user/pass string.

The UTF-8 strings are derived from strings passed through the
module.execute RPC call.
Merge branch 'encoding_fix' of git://github.com/rsmudge/metasploit-framework