Add Rack::Lint to DebugLocks tests

This adds additional test coverage to HostAuthorization to validate that
its behavior conforms to Rack SPEC.

This fixes the following two issues in the reponse returned by
DebugLocks:
- Rack::Lint::Error: uppercase character in header name
  Content-{Type/Length}
- Rack::Lint::Error: a header value must be a String or Array of
  Strings, but the value of 'content-length' is an Integer

actionpack/lib/action_dispatch/middleware/debug_locks.rb

@@ -99,8 +99,8 @@ def render_details(req)
           msg << "\n#{info[:backtrace].join("\n")}\n" if info[:backtrace]
         end.join("\n\n---\n\n\n")
 
-        [200, { "Content-Type" => "text/plain; charset=#{ActionDispatch::Response.default_charset}",
-          "Content-Length" => str.size }, [str]]
+        [200, { Rack::CONTENT_TYPE => "text/plain; charset=#{ActionDispatch::Response.default_charset}",
+                Rack::CONTENT_LENGTH => str.size.to_s }, [str]]
       end
 
       def blocked_by?(victim, blocker, all_threads)

actionpack/test/dispatch/debug_locks_test.rb

@@ -32,7 +32,9 @@ def test_render_threads_status
   private
     def build_app
       @app = self.class.build_app do |middleware|
+        middleware.use Rack::Lint
         middleware.use ActionDispatch::DebugLocks
+        middleware.use Rack::Lint
       end
     end
 end