Avoid mutating header objects passed as arguments

[wemrysi]

Currently, Frame.toString, Client.connect, Client.send, Client.subscribe, Client.ack and Client.nack mutate the headers argument they are invoked with. This causes very subtle bugs in client code as these objects are modified without the caller's knowledge.

Instead, a copy of the headers should be made for the purpose of modification, leaving the original in tact.

[Forusim]

Noticed the same issue.
My workaround:

function headers() { return { persistent: true }; }

client.subscribe('/queue/test', on_message, headers());