RestTemplateBuilder's requestFactory override makes it incompatible with MockRestServiceServer

[snicoll]

In #17091 we've moved some of the customizations of RestTemplateBuilder from a regular interceptor to a wrapping ClientHttpRequestFactory. This makes the created RestTemplate incompatible with MockRestServiceServer as the latter will override the ClientHttpRequestFactory when it binds a rest template to a mock server.

I am not sure how we could avoid that really as the purpose of the factory is to create a request and it must be mocked.

I discovered this as part of testing a component that valides the proper Authorization header is sent. Because the factory is overridden, this header isn't processed at all anymore when managed by MockRestServiceServer.

[snicoll]

Duplicate of #17885