Don't raise if no variable found when using context.key?

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.

lib/liquid/context.rb

@@ -184,7 +184,7 @@ module Liquid
     end
 
     def key?(key)
-      self[key] != nil
+      find_variable(key, raise_on_not_found: false) != nil
     end
 
     def evaluate(object)