Support duplicated SignedMessages in Cdm.set_service_certificate

Fixes #41

Seems some services like TF1 (France) returns a SignedMessage twice in one response body by mistake, resulting in a partial parse decoding error as pywidevine doesn't expect the parsed-then-serialized data to differ from the received data.

This workaround checks if the parsed-then-serialized data is in the received data multiple times without any leftover data. If there's no leftover data it considers it safe to continue.

pywidevine/cdm.py

@@ -222,7 +222,14 @@ def set_service_certificate(self, session_id: bytes, certificate: Optional[Union
 
         try:
             signed_message.ParseFromString(certificate)
-            if signed_message.SerializeToString() == certificate:
+            if (
+                signed_message.SerializeToString() == certificate or
+                # See https://github.com/devine-dl/pywidevine/issues/41
+                all(
+                    bytes(chunk) == signed_message.SerializeToString()
+                    for chunk in zip(*[iter(certificate)] * len(signed_message.SerializeToString()))
+                )
+            ):
                 signed_drm_certificate.ParseFromString(signed_message.msg)
             else:
                 signed_drm_certificate.ParseFromString(certificate)