Add block support for the mail_to helper

This one has caught me out a couple of times recently, since I use the block version of the `link_to` helper quite frequently. This pull request adds block support to the `mail_to` helper so that it works similarly to the `link_to` helper.

A similar update should probably also be made to the actionview-encoded_mail_to gem. I'm happy to submit another pull request to https://github.com/reed/actionview-encoded_mail_to for that change.