R5 BaseReference: copyValues

[brett hanson]

In my Java application which uses Hapi FHIR version 8.4.0, I have created a servlet class for R5 which extends com.rp.vna.fhir.r5.servlet.RestfulServer. Users of this application will issue search requests which will return a Bundle containing a DiagnosticReport and Patient entries.

When creating a org.hl7.fhir.r5.model.DiagnosticReport resource, I am invoking the copy() method. However, the DiagnosticReport.subject:Patient resource is not getting copied and is null. Below is a test class which fails as
diagnosticReport.copy().getSubject().getResource() is null.
```java
import java.util.List;
import org.hl7.fhir.r5.model.DiagnosticReport;
import org.hl7.fhir.r5.model.HumanName;
import org.hl7.fhir.r5.model.Patient;
import org.hl7.fhir.r5.model.Reference;
import org.junit.jupiter.api.Test;

public class R5ReferenceCopyTest {

    @Test
    public void r5ReferenceTest() {
        Patient patient = new Patient();
        patient.setId("222");
        patient.setName(List.of(new HumanName().setFamily("Doe").addGiven("John")));

        Reference reference = new Reference();
        reference.setType(Patient.class.getSimpleName());
        reference.setResource(patient);

        DiagnosticReport diagnosticReport = new DiagnosticReport();
        diagnosticReport.setSubject(reference);

        assert diagnosticReport.copy().getSubject().getResource() != null;
    }
}
```

When I test this same code using only R4 resources, the test will succeed as diagnosticReport.copy().getSubject().getResource() is not null. Comparing the class signatures of org.hl7.fhir.r5.model.BaseReference and org.hl7.fhir.r4.model.BaseReference, I see that in R5 BaseReference now extends DataType and not Element (as it did in R4). However, the copyValues() method in org.hl7.fhir.r5.model.BaseReference is taking in a Element parameter and not a DataType. I am wondering if this is intentional or a bug?