[PATCH] Body length not cast as bytes

[toxinu]

This bug only occur with Python 3 (tested with 3.4).
When building signed headers with `length=True`, `sign` crash.

Test that show bug:

```
    def test_add_body_length(self):
        sig = dkim.sign(
            self.message, b"test", b"example.com", self.key, length=True)
        msg = email.message_from_string(self.message)
        self.assertIn('\n l=%s' % len(msg.get_payload() + '\n'), sig)
        res = dkim.verify(sig + self.message, dnsfunc=self.dnsfunc)
        self.assertTrue(res)
```

======================================================================
ERROR: test_add_body_length (dkim.tests.test_dkim.TestSignAndVerify)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/glehee/Documents/Repositories/dkimpy/dkim/tests/test_dkim.py", line 93, in test_add_body_length
    self.message, b"test", b"example.com", self.key, length=True)
  File "/home/glehee/Documents/Repositories/dkimpy/dkim/__init__.py", line 620, in sign
    return d.sign(selector, domain, privkey, identity=identity, canonicalize=canonicalize, include_headers=include_headers, length=length)
  File "/home/glehee/Documents/Repositories/dkimpy/dkim/__init__.py", line 481, in sign
    sig_value = fold(b"; ".join(b"=".join(x) for x in sigfields))
  File "/home/glehee/Documents/Repositories/dkimpy/dkim/__init__.py", line 481, in <genexpr>
    sig_value = fold(b"; ".join(b"=".join(x) for x in sigfields))
TypeError: sequence item 1: expected a bytes-like object, int found

You can found this work on our github branch here:

https://github.com/oasiswork/dkimpy/commit/b103863a54a420a74584a00754740f606198e6f2

[Scott Kitterman (kitterman)]

Fixed in revision 134. Will be included in 0.6.2.

[Scott Kitterman (kitterman)]

2017-05-30 Version 0.6.2
    - Fixed problem with header folding that caused the first line to be
      folded too long (Updated test test_add_body_length since l= tag is no
      longer at the beginning of a line)
    - Fixed python3.4 string interpolation issue
    - Fix some byte casting issues & typos
    - Add test case for verification when should headers are signed
    - Check and update references:
      * Replace RFC 4871 withRFC 6376
      * Replaace RFC 3447 with RFC 8017
      * Add mention of DCRUP working group addressing key length issues