Override transport.MultipartForm UploadFileSize limit

### What happened?

`handler.NewDefaultServer` adds a default `MultipartForm` transport which limits the UploadFileSize to 32MB. If I would like to override this file size limit, I think I would do something like, according to https://github.com/99designs/gqlgen/blob/master/example/fileupload/server/server.go#L29

```
server := handler.NewDefaultServer(...)
server.AddTransport(transport.MultipartForm{
MaxUploadSize: 100 << 20,
})
```

but I think this function will return the first `transport.MultipartForm`
https://github.com/99designs/gqlgen/blob/master/graphql/handler/server.go#L87

So the file size limit is not overrided

### What did you expect?

Should allow later transport overrides previous ones

### Minimal graphql.schema and models to reproduce

### versions
- `gqlgen version`: v0.13.0
- `go version`: go1.16.3
- dep or go modules?