Ensure close is called on the response body no matter what

Another fallout from #2809 is that
in some cases the `res_body.close` wasn't called because some previous code
raised.

For Rails apps it means CurrentAttributes and a few other important
states aren't reset properly.

This is being improved on the Rails side too, but I believe it would
be good to harden this on the puma side as well.

lib/puma/request.rb

@@ -171,11 +171,16 @@ def handle_request(client, lines, requests)
         end
 
       ensure
-        uncork_socket io
-
-        body.close
-        client.tempfile.unlink if client.tempfile
-        res_body.close if res_body.respond_to? :close
+        begin
+          uncork_socket io
+
+          body.close
+          client.tempfile.unlink if client.tempfile
+        ensure
+          # Whatever happens, we MUST call `close` on the response body.
+          # Otherwise Rack::BodyProxy callbacks may not fire and lead to various state leaks
+          res_body.close if res_body.respond_to? :close
+        end
 
         after_reply.each { |o| o.call }
       end