Optimize sending requests without a body in RestClient and WebClient with Reactor Netty

[rstoyanchev]

The way we use Reactor Netty when there is no body makes it look to Reactor Netty as if we might send a body, and it causes an extra DATA frame to be sent. We should update our request implementations for Reactor Netty to make it easier to execute the request more optimally.

This was originally raised in reactor/reactor-netty#3524, and there is a change on the Reactor Netty side, see reactor/reactor-netty#3526, to do what they can on their side, but it doesn't address all cases, and we really should improve on our end too.