Fixed #13694 -- Made SafeMIMEText's constructor compatible with MIMEText.

https://code.djangoproject.com/ticket/13694
What's the rationale for defaulting charset to 'us-ascii'? Given the way the default is calculated in cpython, it seems like this could result in a behavior change when we remove `SafeMIMEText.__init__()` when our workarounds are no longer needed.