Improve configuration of HttpMessageConverters in RestClient and RestTestClient

[rstoyanchev]

At the moment, the new Consumer<HttpMessageConverters.ClientBuilder> method creates a throwaway builder, and uses it to replace an internal list of converters.

We should change this to having HttpMessageConverters.ClientBuilder instead of a list, and each call with aConsumer would operate on the same builder instance.

The existing Iterable<HttpMessageConverter<?>> can turn off default registrations, and insert provided ones, which would be documented so it's clear how it works.