Ensure mailer view path is loaded when it's assigned. Path#[] raises …

…if it isn't loaded.

actionmailer/lib/action_mailer/base.rb

@@ -427,7 +427,7 @@ def register_template_extension(extension)
 
       def template_root=(root)
         root = ActionView::PathSet::Path.new(root) if root.is_a?(String)
-        write_inheritable_attribute(:template_root, root)
+        write_inheritable_attribute(:template_root, root.load)
       end
     end
 

actionpack/lib/action_view/paths.rb

@@ -42,6 +42,7 @@ def eql?(path)
       end
 
       def [](path)
+        raise "Unloaded view path! #{@path}" unless @loaded
         @paths[path]
       end
 
@@ -51,6 +52,7 @@ def loaded?
 
       def load
         reload! unless loaded?
+        self
       end
 
       # Rebuild load path directory cache