Fix pgp signing failure with Enigmail 1.5.1 on quick reply

hashAlgorithm was removed since Enigmail 1.5.1.
http://sourceforge.net/p/enigmail/source/ci/00690e1464447d8c5f0d67bf629c24b36c3b1be6/
Merge pull request #722 from shirosaki/enigmail

Fix pgp signing failure with Enigmail 1.5.1 on quick reply