Previously if you set `strict_variables` to `true` on the context using
`key?('key_name')` would raise a `Liquid::UndefinedVariable` error.
Raising this error makes sense if you're trying to access the variable
directly with something like `context['key_name']` but by using `key?`
you're safely checking if it exists first.
You should be able to enable `strict_variables` and use `key?` in
combination with each other to ensure code safety.
The YARD liquid gem now supports specifying named parameters. For the
core liquid tags and filters this is the only object I could find that
needed to be updated.
