Fix Malformed header bug

[Uzlopak]

This adapts the bugfix and improvements from @RolandHeinze
mscdex/dicer#22

To see that the bug exists you have to checkout the commit "fix malformed header bug, red phase", which is prepatched phase with added unit test.
Afterwards the above patch was adapted to the codebase in "fix malformed header bug, green phase". The unit test is then green.

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

[kibertoad]

What was the intention of this rewrite?

[Uzlopak]

I think this is the patch?

[Uzlopak]

Do you see a potential regression?

[kibertoad]

I don't understand neither the original code, nor the fix, which is why the change makes me pause. I guess I would need to reread carefully both the original and the change, to fully understand what is happening in both scenarios.

[Uzlopak]

https://www.rfc-editor.org/rfc/rfc2616#section-2.2

   HTTP/1.1 header field values can be folded onto multiple lines if the
   continuation line begins with a space or horizontal tab. All linear
   white space, including folding, has the same semantics as SP. A
   recipient MAY replace any linear white space with a single SP before
   interpreting the field value or forwarding the message downstream.

And for this part there is the test "Folded values" in "dicer-headerparser". If it was wrong the test there would fail.

[kibertoad]

What is the perf impact of this change?

[Uzlopak]

No impact on performance

> @fastify/busboy@1.0.0 bench:dicer
> node deps/dicer/bench/dicer-bench-multipart-parser.js

2439.02 mb/sec
2127.66 mb/sec
4000.00 mb/sec
3846.15 mb/sec
4000.00 mb/sec
4166.67 mb/sec
4000.00 mb/sec
3846.15 mb/sec
4166.67 mb/sec
4000.00 mb/sec