Some of the docs got a little out of date since 3.0.0, and in a couple of cases while updating them I noticed typos or other spots where it no longer behaved the way I expected. In particular, the parsing fallbacks for mixed documents were inconsistent and didn't recognize everything they should.

I also decided to add some parameter validation to Request::deliver after writing in the docs that it would throw an exception if you didn't pass a subscriptionObject as either the default or as an override to deliver. It probably would have thrown an exception in either of those cases, but it might have also just run off the end of a vector iterator or dereferenced nullptr, neither of which would have been particularly easy to diagnose. This way seems a little friendlier to me.

The last change worth mentioning was updating all of the https://facebook.github.io/graphql/June2018/ references in code or documentation to point to their new home at http://spec.graphql.org/June2018/.