title: python protobuf can't deserialize message
body: <p>Getting started with protobuf in python I face a strange issue:</p>
  
  <p>a simple message proto definition is:</p>
  
  <pre><code>syntax = "proto3";
  package test;
  
  message Message {
    string message = 1;
    string sender = 2;
  }
  </code></pre>
  
  <p>generated via <code>protoc -I . --python_out=generated  message.proto</code> and accessed in Python like:</p>
  
  <pre><code>from generated.message_pb2 import Message
  </code></pre>
  
  <p>Then I can construct a message</p>
  
  <pre><code>m = Message()
  m.sender = 'foo'
  m.message = 'bar'
  
  print(str(m))
  </code></pre>
  
  <p>but de-serializing will not return a result</p>
  
  <pre><code>s_m = m.SerializeToString()
  print(s_m) # prints fine
  a = m.ParseFromString(s_m)
  a.foo #fails with error - no attributes deserialized
  </code></pre>