Map field is generated as repeated

I have a protobuf message I have created:

```
message Message {
  ...
  map<string, string> properties = 6;
}
```

But when running the script on that message, the outcome generated is:

```
message Message {
  ...
  repeated PropertiesEntry properties = 6;

  message PropertiesEntry {
    string key = 1;
    string value = 2;
  }
}
```

Instead of map