Silence the MultiJson deprecation warning in every test run

sinatra/json defaults its :json_encoder to MultiJson and calls
.encode on it, which newer multi_json versions deprecate in favor of
a constant this app has no other use for (sinatra-contrib 4.2.1, the
current release, still calls the old one -- not something fixable by
bumping a version). Every use of the json helper here is a plain
hash/array, so pointing :json_encoder at stdlib JSON instead sidesteps
multi_json entirely.

app.rb

@@ -16,6 +16,13 @@ module Maptool
     set :public_folder, File.join(__dir__, "public")
     set :views, File.join(__dir__, "views")
 
+    # sinatra/json defaults to MultiJson, whose `encode` (what
+    # sinatra-contrib 4.2.1 actually calls) is deprecated in newer
+    # multi_json versions in favor of a constant this app has no other use
+    # for — stdlib JSON already does the same job and every use of the
+    # `json` helper below is a plain hash/array, nothing MultiJson-specific.
+    set :json_encoder, JSON
+
     # A persistent secret across restarts/redeploys — otherwise every deploy
     # would invalidate every logged-in session's cookie (see
     # docs/deployment.adoc). Falls back to a random one so the app still