Fix Error 500 in API when accessing a group that has an avatar

Closes #2340

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 7.14.1 (unreleased)
   - Only include base URL in OmniAuth full_host parameter (Stan Hu)
+  - Fix Error 500 in API when accessing a group that has an avatar (Stan Hu)
 v 7.14.0
   - Fix bug where non-project members of the target project could set labels on new merge requests.

app/models/group.rb

@@ -17,6 +17,7 @@
 require 'file_size_validator'
 class Group < Namespace
+  include Gitlab::ConfigHelper
   include Referable
   has_many :group_members, dependent: :destroy, as: :source, class_name: 'GroupMember'

spec/requests/api/groups_spec.rb

@@ -7,7 +7,8 @@
   let(:user2) { create(:user) }
   let(:user3) { create(:user) }
   let(:admin) { create(:admin) }
-  let!(:group1) { create(:group) }
+  let(:avatar_file_path) { File.join(Rails.root, 'spec', 'fixtures', 'banana_sample.gif') }
+  let!(:group1) { create(:group, avatar: File.open(avatar_file_path)) }
   let!(:group2) { create(:group) }
   before do