Fix standardrb offenses

> standard: Use Ruby Standard Style (https://github.com/standardrb/standard)
>   Gemfile:11:9: Layout/CaseIndentation: Indent `when` as deep as `end`.
>   Gemfile:13:9: Layout/CaseIndentation: Indent `when` as deep as `end`.
>   route_downcaser.gemspec:18:80: Layout/LineContinuationSpacing: Use one space in front of backslash.
>   route_downcaser.gemspec:19:77: Layout/LineContinuationSpacing: Use one space in front of backslash.
>   route_downcaser.gemspec:23:3: Gemspec/DeprecatedAttributeAssignment: Do not set `test_files` in gemspec.
> standard: Run `standardrb --fix` to fix up to 5 problems.

Gemfile

@@ -8,12 +8,12 @@ gemspec
 # Enable testing of multiple rails versions
 rails_version = ENV["RAILS_VERSION"] || "default"
 rails = case rails_version
-        when "main"
-          {github: "rails/rails", branch: "main"}
-        when "default"
-          ">= 6.1.0"
-        else
-          "~> #{rails_version}"
+when "main"
+  {github: "rails/rails", branch: "main"}
+when "default"
+  ">= 6.1.0"
+else
+  "~> #{rails_version}"
 end
 gem "rails", rails
 ###############################################

route_downcaser.gemspec

@@ -15,12 +15,11 @@ Gem::Specification.new do |s|
   s.homepage = "https://github.com/carstengehling/route_downcaser"
   s.summary = "Makes routing in Rails case-insensitive"
   s.description =
-    "This gem hooks into the Rack middleware of Rails. This way all paths are "\
-    "downcased before dispatching to Rails' routing mechanism. Querystring "\
+    "This gem hooks into the Rack middleware of Rails. This way all paths are " \
+    "downcased before dispatching to Rails' routing mechanism. Querystring " \
     "parameters are not changed in any way."
 
   s.files = Dir["{lib}/**/*"] + ["MIT-LICENSE", "Rakefile", "README.rdoc"]
-  s.test_files = Dir["test/**/*"]
   s.add_runtime_dependency "activesupport", ">= 3.2"
   s.add_development_dependency "standard"
 end