Liquid::Drop: Expect @context to always be set

[tjoyal]

Liquid::Drop to always assume @context is set.

While this is the direction I think we need to take, I'm not sure this is a safe change to introduce yet until further exploration of reports regarding issues with @context being absent or reset.

I think I would still go ahead and extract these tests as a pre-PR as they better reflect the production behavior and desired code coverage.

Related to https://github.com/Shopify/liquid/issues/1205 (Execution paths missing @context)

[tjoyal]

if we want to be more cautious about breaking changes, then we could have a deprecation warning in Liquid::Drop#liquid_method_missing if @context is nil

👍

Since it is very hard to test for these things, it might be a good path forward to move fast and safely.

I want to iron out as much as possible this is not an issue from the gem itself and then I'll update and send it in review.