Drop Proxy-Connection header

[slandelle]

According to Wikipedia, Proxy-Connection header is a kind of misunderstanding/error in old proxy implementations. If that's the case, the Netty provider shouldn't forcefully add it.

[cngddflzw]

Hi, how's this issue going?

[slandelle]

@jfarcand @rlubke This Proxy-Connection header is only used in JDK and Netty providers. Should we drop it, or make it a common disable by default feature?

[jfarcand]

@slandelle OK drop it and document. I wonder how many clients could be broken by that....