Fix Flask-Mail hack to make pass unit test.

This commit fixes the TODO item in the test_web.py file that had
a typo in the config variable MAIL_DEFAULT_SENDER (it was written DEFAULT_MAIL_SENDER).
Merge pull request #477 from PyBossa/issue-flask-mail-unittests

Fix Flask-Mail hack to make pass unit test.
Merge branch 'master' into culttag-redux