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 used by method_missing to implement a has_key?
method, and uses that method in respond_to?.
Merge pull request #18 from pwnall/fix_burst

Fix BurstStruct's respond_to? for string keys.