Description

The default params are overwritten after each call to the .send method

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

Bug Report

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Reproducible in:

@slack/client version: 4.1

node version: 8.10

OS version(s):

Steps to reproduce:

import { IncomingWebhook } from '@slack/client';

let incomingHook = new IncomingWebhook(process.env.SLACK_HOOK_URL, {
  channel: 'default-channel',
});

incomingHook.send({
  channel: 'alternative-channel',
  text:    'Hello World!'
});

incomingHook.send('Hello Slack!');

Expected result:

The second message is sent to the default-channel.

Actual result:

The second message has been sent to the alternative-channel