No support for body as URLSearchParams

[azhiv]

Description

The native fetch implemented in a browser (along with node-fetch) provide a complex behaviour when processing body. Namely, in case body is a URLSearchParams object, it adds this header 'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8' to the request and also serializes the corresponding form data.
The problem is that when intersecting the native fetch, Polly doesn't handle this case sending an empty body and a missing type header. Here's a link to the spec.

Shareable Source

const args = {
  username: 'username',
  password: 'password',
};

fetch('/api/v1/user/login', {
  method: 'POST',
  headers: {},
  body: new URLSearchParams(args),
})

The request above is incorrectly handled by Pollyjs, here's the har content for this request:

"postData": {
  "mimeType": "text/plain",
  "params": []
},

as opposed to what's there when I export Network activity into har in Chrome:

"postData": {
  "mimeType": "application/x-www-form-urlencoded;charset=UTF-8",
  "text": "username=username&password=password",
  "params": [
    {
      "name": "username",
      "value": "username"
    },
    {
      "name": "password",
      "value": "password"
    }
  ]
}

Should Pollyjs also handle this case?