Spring Expose headers from native client request after request is committed

Fixes issues like that, when public API does not expose headers that have been set by connector implementation.

Comment From: sokomishalov

@rstoyanchev Fair enough The problem is that ClientHttpRequest::getHeaders contain only headers that have been applied via WebClient API and don't contain headers applied by an underlying connector, (i.e. "user-agent"). Regarding implementation with restriction on protected fields - should this logic be duplicated to the ReactorClientHttpRequest?

Comment From: sokomishalov

@rstoyanchev totally makes sense, I've reworked it according to your recommendations (netty, apache http 5, jetty). p.s. I've noticed you marked these changes to the 6.x milestone - is there any chance this enhancement to be backported on 5.x ?