TestRestTemplate.withBasicAuth resets custom requestFactory

[larsgrefer]

I'm currently using a RestTemplateCustomizer bean to set a custom request factory for all RestTemplate's:

https://github.com/freefair/okhttp-spring-boot/blob/f0f6dcc0ba685c40ebe15de725c1170307027173/autoconfigure/src/main/java/io/freefair/spring/okhttp/OkHttpRestTemplateAutoConfiguration.java#L22-L27

The TestRestTemplate I get injected into my unit tests also uses this custom request factory, but calling withBasicAuth() returns a new TestRestTemplate-instance which no longer contains the custom request factory but a default one.

Environment:
Spring Boot 2.1.1.RELEASE
OpenJDK 11

Similar issues:
see #7812
see #7441
see #7265

[mbhave]

I think we should fix this but this commit has some history around why a new request factory is selected when a TestRestTemplate with withBasicAuth is created.

[mbhave]

Closing in favor of PR #15982.