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

MSP-11609

Author: David Maloney

spec/lib/msf/ui/command_dispatcher/db_spec.rb

@@ -25,7 +25,7 @@
       context "when a core already exists" do
         before(:each) do
           priv = FactoryGirl.create(:metasploit_credential_password, data: password)
-          pub = FactoryGirl.create(:metasploit_credential_public, username: username)
+          pub = FactoryGirl.create(:metasploit_credential_username, username: username)
           core = FactoryGirl.create(:metasploit_credential_core,
                                     origin: FactoryGirl.create(:metasploit_credential_origin_import),
                                     private: priv,