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.
These can be replaced with:

`it { should have_key(:foo) }`
`it { should have_key('foo') }`