correct testProcessCreationFormHasErrors method's post url

Vet, Owner, Visit. The Visit "aggregate" is a little artificial
but it demonstrates a useful point about not holding on to
references of "parent" (reference data) objects, i.e. the Visit has
an Integer petId, instead of a Pet field. In principle this app is
now almost ready to migrate to multiple services if anyone wanted
to do that.

Merging AbstractClinicServiceTests and ClinicServiceSpringDataJpaTests then using the Spring Boot @DataJpaTest annotation

- remove custom xml view, use springboot builtin xml view
- remove oxm maven dependency
- remove json-simple maven dependency (seems not relevant anymore)
- update vetsXml test using xpath

- In the Jdbc implementation: pets belonging to a visit were not added.
- In the Jpa implementation: query variable was wrong.
- Test case: AbstractClinicServiceTests.shouldFindVisitsByPetId()

RSS is becoming outdated, JSon is a better example

Happens since introduction of new mvc namespace tag for
content-negotiation