Do not take the name length into account on the second line when folding.

Setting namelen could have no effect, my guess is that this was the
original intention of the code.

This results in more efficient usage of lines.

dkim/__init__.py

@@ -403,7 +403,7 @@ def fold(header, namelen=0):
             j = i + 1
         pre += header[:j] + b"\r\n "
         header = header[j:]
-        namelen = 0
+        maxleng = 72
     if len(header) > 2:
         return pre + header
     else:

dkim/tests/test_arc.py

@@ -74,7 +74,7 @@ Y+vtSBczUiKERHv1yRbcaQtZFh5wtiRrN04BLUTD21MycBX5jYchHjPY/wIDAQAB"""
         sig_lines = dkim.arc_sign(
             self.message, b"test", b"example.com", self.key, b"lists.example.org", timestamp="12345")
 
-        expected_sig = [b'ARC-Seal: i=1; cv=none; a=rsa-sha256; d=example.com; s=test; t=12345; \r\n b=2DTLaiAcKznJwwNOWoOG8WBsdTq+/S92TZbURDxkgjGCmsSw8czQiisf02sC92\r\n 0nswz3JItA80l70iguM00onrj3eCe41yRDzB8lQL3kbrDyM+wUewmyhPoifRTsng\r\n t9ELTFrax4kCeHv6SdNz3uJfGYwQc+WCFEchXt3szNTRM=\r\n', b'ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; \r\n d=example.com; s=test; t=12345; h=message-id : \r\n date : from : to : subject : from; \r\n bh=wE7NXSkgnx9PGiavN4OZhJztvkqPDlemV3OGuEnLwNo=; \r\n b=a0f6qc3k9eECTSR155A0TQS+LjqPFWfI/brQBA83EUz00SNxj\r\n 1wmWykvs1hhBVeM0r1kEQc6CKbzRYaBNSiFj4q8JBpRIujLz1qL\r\n yGmPuAI6ddu/Z/1hQxgpVcp/odmI1UMV2R+d+yQ7tUp3EQxF/GY\r\n Nt22rV4rNmDmANZVqJ90=\r\n', b'ARC-Authentication-Results: i=1; lists.example.org; arc=none;\r\n  spf=pass [email protected];\r\n  dkim=pass (1024-bit key) [email protected];\r\n  dmarc=pass\r\n']
+        expected_sig = [b'ARC-Seal: i=1; cv=none; a=rsa-sha256; d=example.com; s=test; t=12345; \r\n b=AwwfB+haHcusCH1o5kKebsCtsGWBa2sW1jJRHPNLRv8e5fuDl8VMYtj/r5LIZRgj+2ubPc\r\n hjLhoj6/Nf3Sv5m3P6iW9nnXM/HV0CLFxD5RjLhNy0Wsi6POBHVQ2KTT85xv7AyBURBv00Np\r\n kgGt+v3g7wtDQ3T+MwquceXqPmRjM=\r\n', b'ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; \r\n d=example.com; s=test; t=12345; h=message-id : date : from : to : \r\n subject : from; bh=wE7NXSkgnx9PGiavN4OZhJztvkqPDlemV3OGuEnLwNo=; \r\n b=a0f6qc3k9eECTSR155A0TQS+LjqPFWfI/brQBA83EUz00SNxj1wmWykvs1hhBVeM0r1kEQ\r\n c6CKbzRYaBNSiFj4q8JBpRIujLz1qLyGmPuAI6ddu/Z/1hQxgpVcp/odmI1UMV2R+d+yQ7tU\r\n p3EQxF/GYNt22rV4rNmDmANZVqJ90=\r\n', b'ARC-Authentication-Results: i=1; lists.example.org; arc=none;\r\n  spf=pass [email protected];\r\n  dkim=pass (1024-bit key) [email protected];\r\n  dmarc=pass\r\n']
 
         self.assertEqual(expected_sig, sig_lines)