component AP gateway: reset stream position after getting payload:

the stream position needs to be reset, as the body may be read again to compute signature
hash.

sat/plugins/plugin_comp_ap_gateway/http_server.py

@@ -1042,6 +1042,9 @@ class HTTPAPGServer(web_resource.Resource):
             )
             request.finish()
             return
+        else:
+            request.content.seek(0)
+
         try:
             if data["type"] == "Delete" and data["actor"] == data["object"]:
                 # we don't handle actor deletion