Fix multipart/form-data being handled as json

[einkoro]

When working with web workers in the browser (or CloudFlare Workers) we do not have access to FormData.getHeaders() to construct requests (example in #121 (comment)) and bent will default to application/json whereas fetch should handle the content type and boundary headers when passing an instance of FormData to it.

This pull request changes bent to skip adding the application/json content-type header and stringifying the body when the body is an instanceof FormData.

[Zoramite]

I've tested this fix locally and it works great. It allows you to send form data using native FormData in the browser without needing an external library.

For example:

const postJSON = bent('json', 'POST');

const formData = new FormData();
// File data from form field, drag and drop, etc.
formData.append('file', fileData);

const response = await postJSON('https://...', formData);

Since the content-type doesn't get set the browser is correctly able to use the FormData object to send the form as a multi-part request in the fetch.

[bill-work-acc]

For anyone who can't wait this PR to be merged, can use npm to install this PR version of bent directly

npm i mikeal/bent#pull/135/head