get_codec modifies its argument

[jeromekelleher]

The get_codec function modifies its argument, popping the "id" key. This can lead to surprising behaviour such as:

config = {"id": "json"}
codec = get_codec(config) # Works
codec = get_codec(config) # fails.

I suggest adding config = dict(config) at the start of the function. The performance implications should be minimal I think.

[alimanfoo]

SGTM, although I think there may be some other code that relies on the "id" having been popped. May need some adjustment. PR welcome.