SendEmailHandler sending mails with incorrect sender (minor fix)

[DennisOng]

In line 71 of handlers.py (in SendEmailHandler) , the condition

if sender != ' ' will match any specified sender, and as a result, the sender email will either be replaced by self.app.config.get('contact_sender') or "%s <no-reply@%s.appspotmail.com>" % (app_id, app_id) ......

eg: If I want to use "hello@%s.appspotmail.com" % (app_id) as the sender email, it will not allow me although it should be a valid sender.

maybe sender == ' ' should raise an exception instead?