test: Bring test_forward_encrypted_to_unencrypted into line with current API

Currently `Chat.send_msg()` modifies the source message and returns another message object
equivalent to the source one. That's how it works in the core and thus in Python bindings too.

Author: iequidoo

python/tests/test_1_online.py

@@ -511,9 +511,11 @@ def test_forward_encrypted_to_unencrypted(acfactory, lp):
 
     lp.sec("ac2: forward message to ac3 unencrypted ")
     unencrypted_chat = ac2.create_chat(ac3)
+    msg_id = msg.id
     msg2 = unencrypted_chat.send_msg(msg)
-    assert not msg2.is_encrypted()
-    assert msg.is_encrypted()
+    assert msg2 == msg
+    assert msg.id != msg_id
+    assert not msg.is_encrypted()
 
 
 def test_forward_own_message(acfactory, lp):