i18n key for mailer subject conflicts with lazy view keys

[jo]

The keys for the mailer subjects are eg:

devise.mailer.confirmation_instructions

This makes it impossible to use lazy keys in mailer views, like

<%=t '.salutation' %>

because this would search for a

devise.mailer.confirmation_instructions.salutation

which produces a conflict.

It would be great to change the keys for the subject to eg.

devise.mailer.confirmation_instructions.subject

Greetings
Johannes

[josevalim]

Make I18n key for mailer compatible with lookup shortcuts, closed by 80895c3

[jo]

wunderful!