Sending with options overwrites extended slack object

[herkyl]

Slack notify overwrites extended object when sending an object. I think that the wanted action is for a new message to get all the extended parameters and only overwrite when you explicitly define something.

Steps to produce:

const slackNotify = require('slack-notify')(slackWebhookURL);

// extend a base slack object
const slack = slackNotify.extend({
  username: 'Slack Notify',
  channel: '#user-events',
  icon_emoji: ':mailbox_with_mail:'
});

// send a message to a specific channel with an emoji
slack({
  text: 'User changed subscription plan',
  icon_emoji: ':money_mouth_face:',
  channel: 'billing',
});

// !!! when I now send this basic event it
// !!! instead goes to #billing channel and shows the :money_mouth_face: emoji
slack('A basic event that should go to the default #user-events channel');

[herkyl]

I think that the bug might happen due to possible misuse of extend.

https://github.com/andrewchilds/slack-notify/blob/master/slack-notify.js#L108

pub.send(_.extend(defaults, options), done);

this overwrites the defaults object, you can fix it like so:

pub.send(_.extend({}, defaults, options), done);