This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
We had a missing formats: string on our render partial that caused logs to
spam when CSS files got 404s.
Due to magic discourse_public_exceptions.rb was actually returning the
correct 404 cause it switched format when rendering the error.
