Make it unnecessary to manually set the type in the Message model.

[eucyt]

Currently, when instantiating a model that inherits from Message, you need to explicitly set the type.

$message = (new TextMessage())
  ->setType(\LINE\Constants\MessageType::TEXT) // this is redundant.
  ->setText('hello!');

However, this value is obvious from the subclass itself, so having to set it manually is redundant.
Let’s make it set automatically.

Two possible approaches:

1. Change the code generation logic so that the constructor automatically sets its own discriminator. (recomended)
2. Make it internally call fromAssocArray.