Fix Erubi on JRuby, document the issue

lib/haml/railtie.rb

@@ -26,7 +26,15 @@ class Railtie < ::Rails::Railtie
           require "haml/sass_rails_filter"
         end
 
-        if defined?(::Erubi) && const_defined?('ActionView::Template::Handlers::ERB::Erubi')
+        # Any object under ActionView::Template will be defined as the root constant with the same
+        # name if it exists. If Erubi is loaded at all, ActionView::Template::Handlers::ERB::Erubi
+        # will turn out to be a reference to the ::Erubi module.
+        # In Rails 4.2, calling const_defined? results in odd exceptions, which seems to be
+        # solved by looking for ::Erubi first.
+        # However, in JRuby, the const_defined? finds it anyway, so we must make sure that it's
+        # not just a reference to ::Erubi.
+        if defined?(::Erubi) && const_defined?('ActionView::Template::Handlers::ERB::Erubi') &&
+            ActionView::Template::Handlers::ERB::Erubi != ::Erubi
           require "haml/helpers/safe_erubi_template"
           Haml::Filters::RailsErb.template_class = Haml::SafeErubiTemplate
         else