Steps to reproduce

1. Create a default mailer layout (mailer.html.erb)
2. Use stylesheet_link_tag in the layout (e.g:<%= stylesheet_link_tag "mailer", media: "all" %>)
3. Attempt to generate an email
4. Observe that the following exception is raised:

ActionView::Template::Error: undefined method `send_early_hints' for nil:NilClass

Expected behavior

The link should be generated without an exception, and without early hints.

Actual behavior

The helper attempts to call request.send_early_hints, which raises an exception because request is nil: https://github.com/rails/rails/blob/v5.2.0.beta2/actionview/lib/action_view/helpers/asset_tag_helper.rb#L143

System configuration

Rails version: 5.2.0.beta2

Ruby version: Ruby 2.4.0