Uses Guzzle Command's built in default support.

This uses Guzzle Command's built in support for default parameters [1]
to replace the built in support.

There are two advantages to this

* avoids reinventing the wheel
* ensures that the defaults are only applied to commands and not to
  other methods that are proxied to the command client (e.g. executeAll
  for batch requests).

[1] https://github.com/guzzle/guzzle-services/blob/master/src/GuzzleClient.php#L32