title: isinstance doesn't work when use reflection of python protobuf v2.6.1
body: <p>I am using dynamic reflection functionality of python protobuf v2.6.1, and have a function like below:</p>
  
  <pre><code># initilization code
  
          des_db_ = descriptor_database.DescriptorDatabase()
          des_pool_ = descriptor_pool.DescriptorPool(des_db_)
  
  
          fdp = descriptor_pb2.FileDescriptorProto.FromString(
              a_pb_module.DESCRIPTOR.serialized_pb)
          des_db_.Add(fdp)
  
  
  def unpack_PB_msg(type_name, pb_msg_str)
          factory = message_factory.MessageFactory(des_pool_)
          msg_class = factory.GetPrototype(des_pool_.FindMessageTypeByName(type_name))
  
          pb_msg = msg_class()
          pb_msg.ParseFromString(pb_msg_str)
  
         return pb_msg
  </code></pre>
  
  <p>But following client code will fail</p>
  
  <pre><code>hello = Hello_msg()
  hello_str = hello.SerializeToString()
  hello2 = unpack_PB_msg(Hello_msg.DESCRIPTOR.full_name, hello_str)
  
  hello3 = Hello_msg()
  hello3.CopyFrom(hello2)# failed here!!!
  </code></pre>
  
  <p>The error message is:</p>
  
  <pre><code>hello3.CopyFrom(hello2)
    File "C:\Localdata\Python27\lib\site-packages\google\protobuf\message.py", line 119, in CopyFrom
      self.MergeFrom(other_msg)
    File "C:\Localdata\Python27\lib\site-packages\google\protobuf\internal\python_message.py", line 971, in MergeFrom
      "expected %s got %s." % (cls.__name__, type(msg).__name__))
  TypeError: Parameter to MergeFrom() must be instance of same class: expected Hello_msg got Hello_msg.
  </code></pre>
  
  <p>It seems CopyFrom fails because isinstance fails.</p>
  
  <pre><code>  def MergeFrom(self, msg):
      if not isinstance(msg, cls):
        raise TypeError(
            "Parameter to MergeFrom() must be instance of same class: "
            "expected %s got %s." % (cls.__name__, type(msg).__name__))
  </code></pre>
  
  <p>When print data type of hello2 and hello3, it seems they are different.</p>
  
  <pre><code>hello2 : &lt;class 'Hello_msg'&gt;
  hello3 : &lt;class 'a_pb_module.Hello_msg'&gt;
  </code></pre>
  
  <p>Is this a protobuf bug? or I made something wrong?</p>