Fixed #23910 -- Added reply_to parameter to EmailMessage

Yes, I would open a new PR(and probably a ticket too) unless there is a reason I don't know about it. In any case, I don't think using `assert`s is a good idea. Try the following code with and without the `-O` option for example:

```py
from django.core.mail.message import EmailMessage

email = EmailMessage(
    'Subject', 'Content',
    'from@example.com',
    'to@example.com')

print(email.message())
```

Without the `-O` option:

```sh
$ DJANGO_SETTINGS_MODULE="wakefield.settings" python x.py
Traceback (most recent call last):
  File "x.py", line 6, in <module>
    'to@example.com')
  File "/home/berker/hacking/django/django/core/mail/message.py", line 227, in __init__
    assert not isinstance(to, six.string_types), '"to" argument must be a list or tuple'
AssertionError: "to" argument must be a list or tuple
```

With the `-O` option:

```sh
$ DJANGO_SETTINGS_MODULE="wakefield.settings" python -O x.py
From nobody Wed Nov 26 18:30:08 2014
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Subject: Subject
From: from@example.com
To: t, o, @, e, x, a, m, p, l, e, ., c, o, m
Date: Wed, 26 Nov 2014 18:30:08 -0000
Message-ID: <20141126183008.1632.71698@rama>

Content
```