Option to Drop support for Proxy-Connection

[Mythra]

Currently, http-parser will always support the: Proxy-Connection header. This header is not standard, and not required for any real legitimate client:

• https://jdebp.eu/FGA/web-proxy-connection-header.html
• https://en.wikipedia.org/wiki/List_of_HTTP_header_fields

are two examples explaining this weird header.

As a result, everything built on top of http-parser implicitly has to support this unstandard header (which can lead to some bugs if you're not expecting this unstandard case to be handled see: https://github.com/envoyproxy/envoy/pull/6749 ),

It'd be great if we had a flag to turn off (or just didn't support at all) this header that isn't part of the standard.

[PiotrSikora]

cc @indutny (also, llhttp seems to handle Proxy-Connection in a similar way).