Call to_liquid in Context invoke

Different approach to https://github.com/Shopify/liquid/pull/403.

Closes https://github.com/Shopify/liquid/issues/227.

@alexcoco, @dylanahsmith

I'm probably missing something... Dylan?
Why do we need to check this?  We call to_liquid unconditionally in [Context#find_variable](https://github.com/Shopify/liquid/blob/f31e3097703ab102a204bbb054c7d1e47332103d/lib/liquid/context.rb#L207)