Fix Mattermost command creation by specifying username

app/models/project_services/mattermost_slash_commands_service.rb

@@ -46,6 +46,6 @@ class MattermostSlashCommandsService < ChatSlashCommandsService
       description: "Perform common operations on: #{pretty_project_name}",
       display_name: "GitLab / #{pretty_project_name}",
       method: 'P',
-      user_name: 'GitLab')
+      username: 'GitLab')
   end
 end

spec/models/project_services/mattermost_slash_commands_service_spec.rb

@@ -36,7 +36,7 @@ describe MattermostSlashCommandsService, :models do
               description: "Perform common operations on: #{project.name_with_namespace}",
               display_name: "GitLab / #{project.name_with_namespace}",
               method: 'P',
-              user_name: 'GitLab' }.to_json).
+              username: 'GitLab' }.to_json).
             to_return(
               status: 200,
               headers: { 'Content-Type' => 'application/json' },