Shouldn't rails.gemspec include the MIT-LICENSE file in the specification files?
The rails.gemspec only has the README in the Gem Specification's files:
s.files = ["README.md"]
Some projects include every document file or markdown file in the project's root directory to this setting. I don't think that's necessary, but usually the changelog, readme and license should be there. For rails gem, there's no changelog which makes sense but we do have the MIT-LICENSE file.
I can create a PR for this but the Contributing guideline mention it doesn't generally accept changes that "do not add anything substantial to the stability, functionality or testability of Rails".