Error with R exporter

[murphyke]

I was just testing with harvest-openmrs and noticed that the R export fails with:

AttributeError at /api/data/export/r/
'NoneType' object has no attribute 'coded_labels'

At line 50 of query/pipeline.py, the primary key with a raw formatter is prepended to the exporter. At line 75 of _r.py, f['field'] is accessed, which is None, not a DataField instance ....