Support Fastify
Wondering if you would be keen to make a small change to `graphqlUploadExpress` on how it checks if a request is a multipart request. 

Currently the code is using:

```javascript
if (!request.is('multipart/form-data')) return next();
```

as the check for multipart requests and i know the `.is` function is unique to express.

However the rest of the code pretty much works well with fastify and I only had to change that line to:

```javascript
if (!(/^multipart\/form-data/.test(request.headers['content-type']))) return next();
```

If we make the change to use plain javascript instead of a specific express function, it's a little more compatible with frameworks somewhat compatible with express. Alternatively, we could create an entire new file called `graphlUploadFastify` but I think that'll just be duplicating code.

Let me know and if you're fine with the change and I will create a PR for it