Fix export_csv functionality

5862fd13 introduced a change that caused the
`export_csv` controller action to try to load an issue from a missing :id
parameter.

app/controllers/projects/issues_controller.rb

@@ -10,7 +10,7 @@ class Projects::IssuesController < Projects::ApplicationController
 
   before_action :redirect_to_external_issue_tracker, only: [:index, :new]
   before_action :module_enabled
-  before_action :issue, except: [:index, :new, :create, :bulk_update]
+  before_action :issue, except: [:index, :new, :create, :bulk_update, :export_csv]
 
   # Allow write(create) issue
   before_action :authorize_create_issue!, only: [:new, :create]