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

https://code.djangoproject.com/ticket/13694
It's the default value of `_charset` in `MIMEText.__init__`: https://hg.python.org/cpython/file/41ea764e8321/Lib/email/mime/text.py#l18

If I read the code correctly, if you pass third a parameter to `SafeMIMEText.__init__` nothing is changed. If you don't, the following line will raise a `TypeError`:

```py
    else:
        MIMEText.__init__(self, text, subtype, charset)
```

Django always uses it with 3 parameters and it looks like a semi-undocumented class. So, I think I can just change the parameter names, or we can close the ticket as "wontfix".