0.21.2 has a breaking change with pre-stringified JSON request body

[kawanet]

Describe the bug

Since commit 5ad6994 in PR #3688 released with axios version 0.21.2,
it has a breaking change which forces to stringify the body even when pre-stringified in app.

To Reproduce

const axios = require("axios");
const body = "{}"; // pre-stringified JSON
const options = {headers: {"content-type": "application/json"}};
axios.post("http://127.0.0.1:3000/", body, options);

• With 0.21.1, it sends a 2 byte request body {} to the server.
• With 0.21.2, it sends a 4 byte request body "{}" to the server.

My app have a big post data which is already built by my app.
Axios should just send the body content pass through to the server.

Expected behavior

• keep 0.21.1's behavior

Environment

• Axios Version 0.21.1 and 0.21.2
• Node.js Version 14.17.0

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Thanks

[kawanet]

thanks bot. #4018 just copied.