[PATCH] Fix missing SHOULD header bytes representation

[toxinu]

This bug only occur with Python 3 (tested with 3.4).

`references` header field in DKIM.SHOULD is not represented as bytes.

Test that show bug:

```
def test_simple_signature(self):
    # A message verifies after being signed with SHOULD headers
    for header_algo in (b"simple", b"relaxed"):
        for body_algo in (b"simple", b"relaxed"):
            sig = dkim.sign(
  self.message, b"test", b"example.com", self.key,
      canonicalize=(header_algo, body_algo),
      include_headers=(b'from',) + dkim.DKIM.SHOULD)
     res = dkim.verify(sig + self.message, dnsfunc=self.dnsfunc)
     self.assertTrue(res)
```

======================================================================
ERROR: test_simple_signature (dkim.tests.test_dkim.TestSignAndVerify)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/glehee/Documents/Repositories/dkimpy/dkim/tests/test_dkim.py", line 87, in test_simple_signature
    include_headers=(b'from',) + dkim.DKIM.SHOULD)
  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 471, in sign
    (b'h', b" : ".join(include_headers)),
TypeError: sequence item 20: expected a bytes-like object, str found

You can found this work on our github branch here:

https://github.com/oasiswork/dkimpy/commit/6a4dd61b86f8358d4632c17ea6636651387874c7

[Scott Kitterman (kitterman)]

The underlying issue was fixed in commit 134. Added the new test case based on your patch. Thanks. This will be in version 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