Getting superfluous_charset warnings since v2.4.0

[seratch]

Description

It's late to notice this ... 🤦but, since this commit 863e1d3 that was included in v2.4.0, this library has been sending application/x-www-form-urlencoded;charset=utf-8 for Content-Type request header. However, the ;charset=utf-8 is unnecessary and it has been causing the following warning in API responses.

{
  "warning": "superfluous_charset",
  "response_metadata": {
    "warnings": [
      "superfluous_charset"
    ]
  }
}

To know this more, reading the following blog post would be the best:

You’ve sent Slack a message we totally understood, but we had to make some assumptions about the request to service it:
{ "ok": true, "warning": "superfluous_charset" }
In this example, the warning indicates that you’ve sent us the right Content-type for the request, application/x-www-form-urlencoded, but you added an unneeded charset attribute.

https://medium.com/slack-developer-blog/well-wishes-and-warnings-in-the-web-api-489456837220

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.

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Bug Report

This issue is reproducable with any API method requests with request body in the application/x-www-form-urlencoded format.

Logs, screenshots, screencast, sample project, funny gif, etc.