Add ability to not to raise error when included field can't be retrieved

[andriiNyzhnik]

I have different widgets. And provided them in index action.
Each widget type, has different/custom data. For that reason, I add different serializers for each widget type, to respond with that custom data.

The issue is that I have to include that data(different for each widget) in index action.
Gem raises an error in such case:

raise JSONAPI::Serializer::InvalidIncludeError.new("'#{attribute_name}' is not a valid include.")

I need to provide a way, to not to raise that error and just omit relationship if it can't be retrieved.

Can we merge it? Or maybe you can suggest me so other solution?

@fotinakis , @mwpastore what do you think guys?

Here is #118

[krainboltgreene]

Seems like a better approach would be to have includes no-op at the Policy level?

[andriiNyzhnik]

Ok. Than we can close issue and PR