FormData undefined is not a function

[cesar-medina-25]

Hello!! I'm doing some testing with MirageJS and React Native, but it is crashing using FormData in requests.
Specifically in the next block of code (pretender.js line 1569) added in this PR #320

if (body instanceof FormData) {
  body.forEach(function (value) {
    if (value instanceof File) {
          totalSize += value.size;
    }
    else {
          totalSize += value.length;
    }
  });
}

Using console log with body returns these values:

{"_parts": [["grant_type", "password"], ["username", "[email protected]"], ["password", "123456"]]}

Then the app returns an error because body doesn't have a forEach function but it is an instanceof FormData:

TypeError: undefined is not a function (near '...body.forEach...')
scheduleProgressEvent@http://localhost:8081/index.bundle?platform=ios&dev=true&minify=false:131702:25

Versions:

Pretender: 3.4.6
Axios: 0.21.1
MirageJS: 0.1.41
React Native: 0.63.4