separator/spacing issue with ARC signature method dkim.arc_sign

[niftylettuce]

The output of this method returns one of these lines:

```
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=forwardemail.net; s=default; t=1596689357; h=content-type : from : to : subject : message-id : date : mime-version : from; bh=UXpJHdHiPAT+mmzadJeo0PJx/eR3NXE8hbV9KyG2Ass=;
```

You can see the extra spacing in between the ":" colon characters in the "h=" term.

Specifically the issue is in two places:

```py
(b'h', b" : ".join(include_headers)),
```

And

```py
(b'h', b" : ".join(include_headers)),
```

It should be ":" instead without the extra spacing.

[Scott Kitterman (kitterman)]

Does this affect verification? I don't think it would.

[Launchpad Janitor (janitor)]

[Expired for dkimpy because there has been no activity for 60 days.]