This commit removes the RouterFunctions.toHandlerMapping method, in
favor of native support for RouterFunctions in @EnableWebFlux
configuration classes.
In order to accomplish this, the HandlerStrategies components has been
repurposed to only be used for the "bare-bones" HttpHandler, while the
(newly introduced) RouterFunctionMapping uses the strategies as exposed
through WebFluxConfigurationSupport.
Furthermore, this commit also introduces support for testing
RouterFunctions without resorting to an application context.
Issue: SPR-15536
01e3561db9