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?.
Version bump
work around rspec `its` deprecation with rspec-its

Was seeing this deprecation message in the rspec output

    Use of rspec-core's `its` method is deprecated. Use the rspec-its gem
    instead.
updating deprecated rspec methods: be_true, be_false
expose BurstStruct::Burst#keys

expose @hash.keys through BurstStruct::Burst#keys