Bug/msp 11609/blank username failure

this spec needs to use the username factory
:metasploit-credential_public factory will randomly
return either a Username or BlankUsername and thus is
not appropriate for when you want tos et an explicit Username.
The :metasploit_credential_username factory should be used for this
instead

VERIFICATION STEPS
- [ ] land https://github.com/rapid7/metasploit-credential/pull/79
- [ ] remove `gem 'metasploit-credential', git: 'https://github.com/rapid7/metasploit-credential.git', branch: 'bug/MSP-11609/blank-username-failure'` from the Gemfile
- [ ] DELETE the line from the framework-db gemspec that has the 'DELETE this' comment
- [ ] uncomment the line above that in the gemspec
- [ ] `bundle install`
- [ ] `bundle update metasploit-credential`
- [ ] `rake spec`
- [ ] VERIFY no failures