don't convert mailer default values to procs

Invoke mailer defaults as procs only if they are procs, do not convert
with to_proc.  That an object is convertible to a proc does not mean it's
meant to be always used as a proc.  Fixes #11533

Conflicts:
	actionmailer/CHANGELOG.md
Merge pull request #11581 from bughit/11533_to_proc_backport

don't convert mailer default values to procs