Spring Cache ServerHttpRequest::getMethod in AbstractServerHttpRequest

ServerHttpRequest.method() is called twice on each HttpMethodPredicate and within CorsUtils.isPreFlightRequest(). Each call has to do a roundtrip all the way into netty and it does a bunch of String hashing to resolve the proper Spring HttpMethod. I moved this work to be done once upon request construction since the field read is cheaper.

Comment From: poutsma

Thanks for leaving a PR.

It seems only fair that we introduce the same enhancement for the other ServerHttpRequest implementations, so I moved your changes to AbstractServerHttpRequest.