Fix 500 error for requests to the settings path for non-configurable plugins
When a non-configurable plugin is specified in a GET or POST to /settings/plugin/:id, a NoMethodError will be raised, and Redmine will respond with status 500.
The attached patch fixes this by rendering a 404 for both GETs and POSTs unless the plugin is configurable. It's based on trunk r11204.
I've also updated and added tests.