Handle missing Content-Type properly

- ContentManager sets default Content-Type even if it was missing in the
  original message.

- Make sure that when Content-Type is missing, copying parameters doesn't
  raise an error.

- Add a unit-test to check that.

lacre/core.py

@@ -209,7 +209,7 @@ def _rewrap_payload(message: EmailMessage) -> MIMEPart:
     wrapper.set_type(message.get_content_type())
 
     # Copy all Content-Type parameters.
-    for (pname, pvalue) in message.get_params():
+    for (pname, pvalue) in message.get_params(failobj=list()):
         # Skip MIME type that's also returned by get_params().
         if not '/' in pname:
             wrapper.set_param(pname, pvalue)

test/modules/test_lacre_core.py

@@ -40,3 +40,17 @@ class LacreCoreTest(unittest.TestCase):
                          'only content and content-type should be copied')
         self.assertEqual(rewrapped.get_content_type(), 'text/plain',
                          'rewrapped part should have initial message\'s content-type')
+
+    def test_payload_wrapping_wo_content_type(self):
+        m = EmailMessage()
+        m.set_payload('This is a payload.\r\n'
+                      + '\r\n'
+                      + 'It has two paragraphs.\r\n')
+        m['Subject'] = 'Source message'
+
+        rewrapped = lacre.core._rewrap_payload(m)
+
+        self.assertFalse('Subject' in rewrapped,
+                         'only content and content-type should be copied')
+        self.assertEqual(rewrapped.get_content_type(), 'text/plain',
+                         'rewrapped part should have initial message\'s content-type')