Add methods to the presenter which expose information about the deprecation of the set

I was a bit unsure about adding the two new fixtures specs `DeprecatedSpec` and `DeprecatedInFavorOfSpec`, but I wanted to have test cases covering those code paths. Maybe there is a better way to do it?

These changes are preparations for a PR that implements CocoaPods/CocoaPods#2180
I see your point. However this is a very idiomatic thing of Ruby and once ones get used to it feels natural. So it would be great if you could change it (also because that would make it consistent to the rest of the codebase)