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.
Don't feel strongly about this, but I might refactor this as:

    let(:struct) { BurstStruct::Burst.new('foo' => 'bar') }
    subject { burst }

    context 'when a key exists' do
      describe '#foo' do
        subject { struct.foo }
        it { should eql('bar') }
      end