RESTWS-940: PersonAttributeResource should not try to hydrate the attribute itself

[ibacher]

Description of what I changed

Previously, PersonAttributeResource1_8#getDisplayString() had two different formats:

1. For attributes without an attribute type, an empty string
2. For concepts, the concept display string
3. For anything else, AttributeType = <AttributeValue>; e.g., Current Location = 4

This PR ensures that getDisplayString() continues to work like 1 and 2 above, but replaces 3 with something more similar to 2, that is, the display string for the value. This means that in the Current Location example above, getDisplayString() would return a string like "Ward 3".

The AttributeType name can be fetched via a custom display, and the raw value can be read via the value property. Nevertheless, this is technically a breaking change.

Issue I worked on

see https://issues.openmrs.org/browse/RESTWS-940

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.

  No? Unsure? -> configure your IDE, format the code and add the changes with git add . && git commit --amend

• I have added tests to cover my changes. (If you refactored
  existing code that was well tested you do not have to add tests)

  No? -> write tests and add them to this commit git add . && git commit --amend

• I ran mvn clean package right before creating this pull request and
  added all formatting changes to my commit.

  No? -> execute above command

• All new and existing tests passed.

  No? -> figure out why and add the fix to your commit. It is your responsibility to make sure your code works.

• My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master

[coveralls]

coverage: 47.118% (-0.008%) from 47.126%
when pulling e4e37d3 on RESTWS-940
into 6bd939a on master.