Add a failing spec to the POST /users API

spec/requests/api/users_spec.rb

@@ -265,6 +265,14 @@ describe API::Users, api: true  do
         expect(response).to have_http_status(409)
         expect(json_response['message']).to eq('Username has already been taken')
       end
+
+      it 'creates user with new identity' do
+        post api("/users", admin), attributes_for(:user, provider: 'github', extern_uid: '67890')
+
+        expect(response).to have_http_status(201)
+        expect(json_response['identities'].first['extern_uid']).to eq('67890')
+        expect(json_response['identities'].first['provider']).to eq('github')
+      end
     end
   end