Rest client is adding extra quotes to payload

[fushinoryuu]

Environment

Node version: 13.12.0
Npm version: 6.14.4
OS and version: Windows 10
typed-rest-client version: 1.5.0

Issue Description

When trying to send a request with a payload other than JSON, the client is not able to recognize this and adds extra quotes since it performs a JSON.stringify() on the payload.

This can be seen when trying to send a request with a body of application/x-www-form-urlencoded type. This is the example payload I'm trying to send: client_id=some_client&grant_type=password&username=some_guy&password=the_password&client_secret=some_secret&scope=api

Expected behaviour

The rest client shouldn't call JSON.stringify() on a payload that doesn't need it.

Actual behaviour

The rest client is adding extra quotes to the payload, making the request fail.

Steps to reproduce

1. Create the following IRequestOptions:

let options = {
    acceptHeader: "application/json",
    additionalHeaders: {
      "Content-Type": "application/x-www-form-urlencoded",
    }
  };

2. Create a url encoded string that will be the payload for the request:

let body = client_id=some_client&grant_type=password&username=some_guy&password=the_password&client_secret=some_secret&scope=api

3. Try to send the request:

let response = await client.create("someurl.com", body, options);

4. If you step through the code, you will see that the client will add extra quotes on this line: https://github.com/microsoft/typed-rest-client/blob/master/lib/RestClient.ts#L110

I will be submitting a PR to try and get around this.

[github-actions]

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