FIX: correct user serializer user method for extended serializer (#8590)

A small fix for Basic User Serializers where some downstream serializers do not correctly set user objects. This caused some issues in certain plugins that depend on the user method to return a user.

app/serializers/basic_user_serializer.rb

@@ -20,6 +20,6 @@ class BasicUserSerializer < ApplicationSerializer
   end
 
   def user
-    object[:user] || object
+    object[:user] || object.try(:user) || object
   end
 end

spec/fabricators/post_action_fabricator.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+Fabricator(:post_action) do
+  post
+  user
+  post_action_type_id PostActionType.types[:like]
+end

spec/serializers/basic_user_serializer_spec.rb

@@ -14,6 +14,14 @@ describe BasicUserSerializer do
       expect(json[:avatar_template]).to eq(user.avatar_template)
     end
 
+    describe 'extended serializers' do
+      let(:post_action) { Fabricate(:post_action, user: user) }
+      let(:serializer) { PostActionUserSerializer.new(post_action, scope: Guardian.new(user), root: false) }
+      it "returns the user correctly" do
+        expect(serializer.user.username).to eq(user.username)
+      end
+    end
+
     it "doesn't return the name it when `enable_names` is false" do
       SiteSetting.enable_names = false
       expect(json[:name]).to eq(nil)