Expose BurstStruct::Burst#keys

It would be nice if we could expose the `#keys` method on the underlying `@hash` for a given `BurstStruct::Burst` instance. (eee2954)

I also updated the usage of some deprecated rspec methods (d35c6a4, b99c179)

Thanks
Since these are booleans, I'd prefer the exact match.  `should be true` and `should be false` are supposed to work.