add the hook for unknown types also to the cython Packer

Author: antocuni

msgpack/_packer.pyx

@@ -176,6 +176,9 @@ cdef class Packer(object):
                 for v in o:
                     ret = self._pack(v, nest_limit-1)
                     if ret != 0: break
+        elif self.handle_unknown_type(o):
+            # it means that obj was succesfully packed, so we are done
+            return 0
         elif self._default:
             o = self._default(o)
             ret = self._pack(o, nest_limit-1)
@@ -195,6 +198,9 @@ cdef class Packer(object):
             self.pk.length = 0
             return buf
 
+    def handle_unknown_type(self, obj):
+        return None
+
     def pack_extended_type(self, typecode, data):
         msgpack_pack_ext(&self.pk, typecode, len(data))
         msgpack_pack_raw_body(&self.pk, data, len(data))