Backport 2.4.2 2.4.4 fixes

Hi, 

This is a backport to the v2.2 branch of the fixes introduced in [2.4.2..2.4.4](https://github.com/mikel/mail/compare/2.4.2...2.4.4) for [CVE: 2012-2139](http://www.osvdb.org/show/osvdb/81631) and [CVE: 2012-2140](http://www.osvdb.org/show/osvdb/81632) for rails 3.0.x users (as ActionMailer 3.0.x depends on ~2.2.x).

Specs passed for me on ruby 1.8.7 but fail on 1.9.3 but those failures were also present in 2.2.19 (does 2.2 not support 1.9.3?).

N.B. 1abd789e996b5305b516d526d515c304fd9a2c19 isn't a complete cherry-pick in that it doesn't include the exim-related fix as that functionality doesn't appear to be in the 2.2 branch.