Return a 404 response when no record is found

The JSON API specification says:
> A server MUST respond with `404 Not Found` when processing a request to fetch a single resource that does not exist

At the moment JSON API Resources returns a 200, with an empty data
object.

Author: MarcelEeken

lib/jsonapi/processor.rb

@@ -101,6 +101,7 @@ def show
                                        include_directives,
                                        find_options)
 
+      fail JSONAPI::Exceptions::RecordNotFound.new(id) if resource_set.resource_klasses.empty?
       resource_set.populate!(serializer, context, find_options)
 
       return JSONAPI::ResourceSetOperationResult.new(:ok, resource_set, result_options)

test/integration/requests/request_test.rb

@@ -25,6 +25,11 @@ def test_large_get
     assert_cacheable_jsonapi_get '/api/v2/books?include=book_comments,book_comments.author'
   end
 
+  def test_get_not_found
+    get "/people/2000"
+    assert_jsonapi_response 404
+  end
+
   def test_post_sessions
     session_id = SecureRandom.uuid