changing Credential == operator

it should no longer raise no method errors when comparing a credential to
an object that doesnt respond to public, private, or realm

# Verification steps
- [ ] Launch `msfconsole`
- [ ] `irb`
- [ ] `cred = Metasploit::Framework::Credential.new`
- [ ] `cred == cred` should return true
- [ ] `cred == 'string'` should return false