Don't error out in Python 3 if include headers is string (LP: #1776775)

dkim/__init__.py

@@ -724,6 +724,12 @@ class DKIM(DomainSigner):
 
     if include_headers is None:
         include_headers = self.default_sign_headers()
+    try:
+        include_headers = [bytes(x, 'utf-8') for x in include_headers]
+    except TypeError:
+        # TypeError means it's already bytes and we're good or we're in
+        # Python 2 and we don't care.  See LP: #1776775.
+        pass
 
     include_headers = tuple([x.lower() for x in include_headers])
     # record what verify should extract

dkim/tests/test_dkim.py

@@ -244,6 +244,17 @@ p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo="""
                 res = dkim.verify(sig + self.message, dnsfunc=self.dnsfunc)
                 self.assertTrue(res)
 
+    def test_string_include(self):
+        # A message can be signed when the include_headers is string
+        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=('from',) )
+                res = dkim.verify(sig + self.message, dnsfunc=self.dnsfunc)
+                self.assertTrue(res)
+
     def test_add_body_length(self):
         sig = dkim.sign(
             self.message, b"test", b"example.com", self.key, length=True)