# File lib/action_mailer/vendor/tmail-1.2.3/tmail/mail.rb, line 270 def []=( key, val ) dkey = key.downcase if val.nil? @header.delete dkey return nil end case val when String header = new_hf(key, val) when HeaderField ; when Array ALLOW_MULTIPLE.include? dkey or raise ArgumentError, "#{key}: Header must not be multiple" @header[dkey] = val return val else header = new_hf(key, val.to_s) end if ALLOW_MULTIPLE.include? dkey (@header[dkey] ||= []).push header else @header[dkey] = header end val end