This is a continuation of #33. Kinda.

Running the following minimal code with Wrangler used to fail with a ReferenceError:

import { S3mini } from '../../dist/s3mini.min.js';

const client = new S3mini({
  accessKeyId: 'AAA',
  secretAccessKey: 'BBBBBBBB',
  endpoint: 'http://localhost:9002/test',
  region: 'auto',
});

client
  .uploadPart('large-file.zip', 'demo-upload-id', '... some data to add ...', 1)
  .then(console.log, console.error);

ReferenceError: Buffer is not defined

This is because Buffer is not part of base ECMAScript but an extension by Node.js. Now that s3mini only relies on Web Crypto APIs, this tries to remove another Node.js dependency and make it possible to use it in more environments.

1. The internal methods of the S3mini class use BodyInit for its request body value, this should always be compatible with fetch().
2. The externally offered methods continue to accept only strings and Buffers. But validation of this checks to make sure that Buffer is a thing, thus skipping these values in other environments.

It might also be possible to solve this by bringing BodyInit all the way out to the methods. But for this first PR I have chosen to keep the user inputs the same.

After this change, Wrangler returns:

Error: Disallowed operation called within global scope. Asynchronous I/O (ex: fetch() or connect()), setting a timeout, and generating random values are not allowed within global scope.

Which suggests it can run all the library code but does not execute the fetch() because I am running it naively.