Properly give defaults for DatabaseSelector options

The initializer receives `nil` for these options when no cofigurations were given:
https://github.com/rails/rails/blob/v6.0.0.rc1/activerecord/lib/active_record/railtie.rb#L91-L97

activerecord/lib/active_record/middleware/database_selector.rb

@@ -35,10 +35,10 @@ module Middleware
     #   config.active_record.database_resolver = MyResolver
     #   config.active_record.database_resolver_context = MyResolver::MySession
     class DatabaseSelector
-      def initialize(app, resolver_klass = Resolver, context_klass = Resolver::Session, options = {})
+      def initialize(app, resolver_klass = nil, context_klass = nil, options = {})
         @app = app
-        @resolver_klass = resolver_klass
-        @context_klass = context_klass
+        @resolver_klass = resolver_klass || Resolver
+        @context_klass = context_klass || Resolver::Session
         @options = options
       end