Fix close handle for client.ws.depth

[bennycode]

I was testing client.ws.depth and noticed that the close handle doesn't work because the cache assignment is done with a forEach execution.

I fixed it by using map (all other cache assignments are also done with map).

This prevents the following error:

TypeError: Cannot read property 'forEach' of undefined

You can test it with the following snippet:

const close = client.ws.depth('ETHBTC', depth => {
  console.log('new data', depth);
  close();
})

[coveralls]

Coverage decreased (-23.7%) to 69.565% when pulling bf91f08 on bennyn:patch-1 into a02d541 on HyperCubeProject:master.

[balthazar]

Nice, thanks