Fixed #7747: Altered EmailMessage such that messages with long subject lines...

Fixed #7747: Altered EmailMessage such that messages with long subject lines don't use tabs in their continutation sequence. Tabs in subjects cause problems with Outlook and Thunderbird. Thanks to Mark Allison <mark.allison@maplecroft.com> for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8483 bcc190cf-cafb-0310-a4f2-bffc1f526a37