Support Windows development environment
I'm having some issues with testing a server built on this package (see [this](https://stackoverflow.com/questions/49771811/postman-ordering-of-fields-in-form-data-request) and [this](https://github.com/postmanlabs/postman-app-support/issues/4461)) and it ultimately boils down to the fact that [the spec](https://github.com/jaydenseric/graphql-multipart-request-spec#multipart-form-field-structure) requires a specific order of the request body parts, which Postman fails to satisfy.

Since this library can be made less strict about that by simply collecting all parts into a cache before applying the middleware logic, I figured I'd try to submit a PR with that fix, but I can't get the tests to run after a clean clone.

```
$ git clone <this repo>
$ cd apollo-upload-server
$ npm install
...
$ npm run test
...
> apollo-upload-server@5.0.0 postbuild C:\Work\OSS\apollo-upload-server
> prettier 'lib/**/*.{mjs,js}' --write

[error] No matching files. Patterns tried: 'lib/**/*.{mjs,js}' !**/node_modules/** !./node_modules/**
```

If I instead run `npm run watch:test`, all tests fail with the error

```
FetchError {
    code: 'ECONNRESET',
    errno: 'ECONNRESET',
    message: 'request to http://localhost:51517/ failed, reason: read ECONNRESET',
    type: 'system',
}
```

which tells me there's more I have to do to run the tests than just start the test runner in this library.

Is there documentation somewhere on how to contribute, what I need to do to get the development environment up and running etc?