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

1 files changed, 1 insertions, 1 deletions

diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index fcdd6747b8..cc3a412221 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -685,7 +685,7 @@ module ActionMailer
       # Call all the procs (if any)
       class_default = self.class.default
       default_values = class_default.merge(class_default) do |k,v|
-        v.respond_to?(:to_proc) ? instance_eval(&v) : v
+        v.is_a?(Proc) ? instance_eval(&v) : v
       end
 
       # Handle defaults