Support webhook default configuration from Slack UI about chef-slack_handler

[nicwaller]

I agree that it would be preferable to be able to use the icon and username defined in the Slack integration, rather than always overriding it.

But it's not enough to fix this in initialize. If the username isn't set to a sane default then it will be nil, and the resultant JSON sent to Slack will look like this.

{
  "username": null,
  "etc": "etc"
}

So the request_body def needs to be patched too, because that's where the JSON request gets built. If config[:username] is nil, it shouldn't be included in the request at all.

[cknowles]

Just working on a pull request for this.

[cknowles]

This actually works with null username by the way, Slack still uses the default set on its side if null is submitted. Will patch it anyway.

[martinb3]

Fixed in #19.