itkNrrdImageIO.cxx: Use MetaDataObjectBase::Print() to write dict values

Change-Id: I22453151609ac98f6657e14f99ec9c7377e6ba72
itkMetaDataObject.txx: Print bool data as "true" or "false"

Change-Id: I8e7fcfc8f7278c1f8aff2bb5b74692c7b346be8c