Spring Provide method with ContextView instead of Context in ServerWebExchangeContextFilter

Use ContextView in ServerWebExchangeContextFilter#get.

• Context is a subclass of ContextView; hence, this is a non-breaking change.
• It makes the convenience method more convenient because Mono#transformDeferredContextual and Mono#deferContextual provide ContentView instance. This avoids the need for a clunky cast or the unnecessary .map(Context::of).

Comment From: singhbaljit

Tagging @rstoyanchev since you wrote the class originally.