Make tests pass in node v15

[michielbdejong]

Skipped the failing ones in master for now, this issue is to fix them

[michielbdejong]

https://travis-ci.com/github/solid/node-solid-server/builds/187838602 failed

[michielbdejong]

Still failing two https://travis-ci.com/github/solid/node-solid-server/jobs/417913431#L1633