Preserve default_url_options overrides in the mailer class

[bcroesch]

We override default_url_options in our mailers so that we can have dynamic hosts in the mailer's URLs. Roughly something like:

class MyMailer < ActionMailer::Base

  def default_url_options
    { host: "#{some_dynamic_subdomain}.example.com" }
  end

end

Going directly through ActionMailer::Base.default_url_options circumvents that override. This change will preserve it, but also should fall back to ActionMailer::Base.default_url_options if no override exists.

[ankane]

Great, thanks @bcroesch!