Fix all linter warnings and set stricter fail level

Author: sferik

Rakefile

@@ -41,7 +41,7 @@ desc("Lint `*.rb(i)`")
 multitask(:"lint:rubocop") do
   find = %w[find ./lib ./test ./rbi -type f -and ( -name *.rb -or -name *.rbi ) -print0]
 
-  rubocop = %w[rubocop --fail-level E]
+  rubocop = %w[rubocop]
   rubocop += %w[--format github] if ENV.key?("CI")
 
   # some lines cannot be shortened