Bug 94

[BitBucket user: Alain Mazy]
[BitBucket date: 2018-06-29.12:51:13]

perform a simple /instances/../modify request with this payload:
```
curl -X POST http://localhost:8042/instances/e294c1b9-64b50db6-4b600ea8-c41de8a4-b7767de5/modify -d '{"Replace" : {"PatientID" : "toto"}, "Force": true}' > modified.dcm
dcmdump modified.dcm | grep "FrameOfReference"

curl -X GET http://localhost:8042/instances/e294c1b9-64b50db6-4b600ea8-c41de8a4-b7767de5/file > original.dcm
dcmdump original.dcm | grep "FrameOfReference"

```

The FrameOfReferenceUID has changed in the modified instance.


Actually, the RelationshipsVisitor is applied and, since the FrameOfReferenceUID is not mentioned in any of the Keep, Remove and Replace, it is "Enabled" and therefore modified.

```
    bool IsEnabled(const DicomTag& tag) const
    {
      return (!that_.IsCleared(tag) &&
              !that_.IsRemoved(tag) &&
              !that_.IsReplaced(tag));
    }
```

When performing simple modification (that is not an Anonymization), we shall probably just not apply the RelationshipVisitor