Use more specific #avatar_icon_for_user.

Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

spec/helpers/projects_helper_spec.rb

@@ -215,7 +215,7 @@ describe ProjectsHelper do
     let(:expected) { double }
     before do
-      expect(helper).to receive(:avatar_icon).with(user, 16).and_return(expected)
+      expect(helper).to receive(:avatar_icon_for_user).with(user, 16).and_return(expected)
     end
     it 'returns image tag for member avatar' do