nonce is set directly in params, kills the ability to retry without rebuilding params

[pjbrinck]

I'm using a pattern of

function callKraken(method, params, next) {
	async.retry(3, next => {
    		kraken.api(method, params, next);
	}, next); 
}

If I rely on Kraken-Api to set the nonce, then all my retries will fail with "API: Invalid nonce".

Work around: I'll set the nonce myself.
Fix suggestion: Kraken-Api does not alter params other than a possible copy of params.