[rails6] Update ActionView::Template::Handlers::RJS.call

In Rails6, there is a deprecation warning with `.call` when working with
template handlers:

    DEPRECATION WARNING: Single arity template handlers are deprecated.
    Template handlers must now accept two parameters, the view object and the
    source for the view object.
    Change:
      >> #<ActionView::Template::Handlers::RJS:0x00007f8ecf616cb8>.call(template)
    To:
      >> #<ActionView::Template::Handlers::RJS:0x00007f8ecf616cb8>.call(template, source)

This borrows from PR 138 in Hamlit that did the same thing:

- https://github.com/k0kubun/hamlit
- https://github.com/k0kubun/hamlit/blob/master/CHANGELOG.md#293---2019-04-09

lib/action_view/template/handlers/rjs.rb

@@ -5,8 +5,9 @@ class RJS
       class_attribute :default_format
       self.default_format = Mime[:js]
 
-      def call(template)
-        "update_page do |page|;#{template.source}\nend"
+      def call(template, source=nil)
+        source ||= template.source
+        "update_page do |page|;#{source}\nend"
       end
     end
   end