add fieldNameSize for multipart

[Uzlopak]

This PR is not ready for merge.

Discussion-points

• Currently it checks for specific rfc-relevant fieldNames and does not apply fieldNameSize on those fieldnames. Should we remove that and just write in the documentation, that we recommend not to limit the fieldNameSize to some unreasonable value? If so it would remove this check for the specific fieldnames and in real world use cases you want to limit the fieldNameSize to something like 255 characters and not 3.
• I assume that like in the added unit test, that "test" is a value and not a key so it should apply the fieldSize instead of the fieldNameSize, right?

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

Actually multipart seems to not respect fieldSize either.