ENH: Use MetaDataObjectBase::Print() to write NRRD dict values

Ported from release-3.20 branch:

800c63f9a5ac8c8021b14cbe565614d182ec11eb
2e53ea09cd5771d45cd025e1379b24d99dee13d4

Change-Id: I51572e0017ecc6e6d9ccb47ef788b80634938e17