Mention solution to common pitfall in "Testing the Web Layer"

[rehevkor5]

Add explanation to https://spring.io/guides/gs/testing-web/ about the causes of HttpMediaTypeNotAcceptableException or HttpMessageNotWritableException when using @SpringBootTest with @AutoConfigureMockMvc, and what to do about it (probably just adding @AutoConfigureWebMvc).

This was suggested in https://github.com/spring-projects/spring-boot/issues/24000#issuecomment-943134815

[Buzzardo]

To encourage the community to get involved, I'll add the help wanted label. It's also a pretty straightforward addition, so I'll also add the good first issue label.

[manumafe98]

Hi I would like to work on this one!

[Buzzardo]

You don't need to tell us that you want to work on it. To quote a certain shoe company, "Just do it!" Then make a pull request to this repository. Thanks. :)