rails/rails — pull request #15445 by manuelmeurer (2014-05-31 15:09:21+00:00)
Title: Update documentation for setting asset_host to a Proc

mention that the request parameter might not be supplied
Relevant test: https://github.com/rails/rails/blob/3051356a8005518392f3414184daf933f5597092/actionmailer/test/asset_host_test.rb#L45