Guard agains nil content type in Hook#pure_html_message?

Prevents premailer from breaking in the unlikely event that content type is `nil`.

This would probably only happen when relaying email from an outside source, and the sender's email client leaves the content type unset. In my case, it just made me realize that I should have been using `:skip_premailer` in that particular mailer. ;) Still, I thought I'd contribute with a fix.