IndifferentHash monkeypatch warning improvements

* Print warning on Rubies that require monkeypatching only
* Print warning to stderr instead of stdout
* Load ActiveSupport during internal testing only
* Add SINATRA_ACTIVESUPPORT_WARNING opt-out environment variable

lib/sinatra/indifferent_hash.rb

@@ -1,21 +1,23 @@
 # frozen_string_literal: true
-if !$LOAD_PATH.grep(%r{gems/activesupport}).empty? && $LOADED_FEATURES.grep(%r{active_support/core_ext/hash}).empty?
-  puts <<-EOF
+if !Hash.method_defined?(:slice) && !$LOAD_PATH.grep(%r{gems/activesupport}).empty?
+  if ENV['APP_ENV'] == 'test' && File.basename(File.dirname(ENV['BUNDLE_GEMFILE'])) == 'sinatra'
+    # Some extensions get loaded during internal testing (e.g. by RABL and our
+    # RABL test) that we have no control over, but we need them to load
+    # *before* IndifferentHash, so we'll do it preemptively here.
+    #
+    # Newer Rubies have these methods built-in, so the extensions are no-ops.
+    require 'active_support/core_ext/hash/conversions'
+    require 'active_support/core_ext/hash/slice'
+    require 'active_support/core_ext/hash/keys'
+  elsif ENV['SINATRA_ACTIVESUPPORT_WARNING'] != 'false'
+    $stderr.puts <<-EOF
 WARNING: If you plan to load any of ActiveSupport's core extensions to Hash, be
 sure to do so *before* loading Sinatra::Application or Sinatra::Base. If not,
 you may disregard this warning.
-  EOF
-end
 
-if ENV['APP_ENV'] == 'test' && !Hash.method_defined?(:slice)
-  # Some extensions get loaded during testing (e.g. by RABL and our RABL test)
-  # that we have no control over, but we need it to load *before*
-  # IndifferentHash, so we'll do it preemptively here.
-  #
-  # Newer Rubies have these methods built-in, so the extensions are no-ops.
-  require 'active_support/core_ext/hash/conversions'
-  require 'active_support/core_ext/hash/slice'
-  require 'active_support/core_ext/hash/keys'
+Set SINATRA_ACTIVESUPPORT_WARNING=false in the environment to hide this warning.
+    EOF
+  end
 end
 
 module Sinatra