Seg6Action.to_json() does not make a valid argument for posting to the nlsdn /action endpoint

Seg6Action.to_json() creates an object with the fields 'mode', 'segments', 'type', and 'matches'. Since the argument to Seg6Action's constructor doesn't take the 'matches' key, an exception is triggered in the server when it tries to construct a Seg6Action from the json-ized value.

Ref: https://gitlab.flux.utah.edu/safeedge/nlsdn/blob/master/src/nlsdn/server/sdn.py#L208