Update e2e.py

Preventing UnicodeDecodeError in case that Umlaute, etc. are in e.g., an images description.

Author: bruderjakob12

threema/gateway/e2e.py

@@ -1297,7 +1297,7 @@ async def pack(self, writer):
     async def unpack(cls, connection, parameters, key_pair, reader):
         # Get payload
         try:
-            content = bytes(reader.readexactly(len(reader))).decode('ascii')
+            content = bytes(reader.readexactly(len(reader))).decode('utf-8')
         except UnicodeError as exc:
             raise MessageError('Could not decode JSON') from exc