Fix BurstStruct's respond_to? for string keys.

The custom struct implementation handles string/symbol keys correctly in
method_missing, but not in respond_to?. This change reuses the (already
factored out) code usd by method_missing in the respond_to
implementation.
I would probably do:

`!find_key(method_name).nil? || super`

It would probably also be worth adding a `has_key?` method then using that:

`has_key?(method_name) || super`