documentation: unmodifable metadata

[qeepcologne]

don't know which version exactly changes that, but after updating, our app got broken:
Unsupported operation: Cannot modify unmodifiable map
Message was unmodifiable long time, but metadata wasn't.

example::
message.metadata?.remove('btn_smile_back');

i have to change that into:

chat.Message updatedMsg = message.copyWith(
  metadata: Map<String, dynamic>.from(message.metadata!)..remove('btn_smile_back'),
);
_chatController.updateMessage(message, updatedMsg);

I'm totally fine with the change, but it should be mentioned in changelog.

[demchenkoalex]

sorry about that, not sure if it was v1 to v2 where I changed everything or some recent dependency upgrade.. I will double check what happened there and maybe cover with tests