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 to set an explicit Username. The :metasploit_credential_username factory should be used for this instead.

VERIFICATION STEPS
- [x] land https://github.com/rapid7/metasploit-credential/pull/79
- [x] remove `gem 'metasploit-credential', git: 'https://github.com/rapid7/metasploit-credential.git', branch: 'bug/MSP-11609/blank-username-failure'` from the Gemfile
- [x] DELETE the line from the framework-db gemspec that has the 'DELETE this' comment
- [x] uncomment the line above that in the gemspec
- [x] `bundle install`
- [x] `bundle update metasploit-credential`
- [x] `rake spec`
- [x] VERIFY no failures