hash-fetch-defaults: fetch(:is_evil, true) to fetch(:is_evil) { true }

In [hash-fetch-defaults](https://github.com/bbatsov/ruby-style-guide#hash-fetch-defaults), the last line is `batman.fetch(:is_evil, true)`. This sort of contradicts the next guideline of preferring the use of a block. 

change `batman.fetch(:is_evil, true)` to `batman.fetch(:is_evil) { true } `