invertIndex fails when no TF/IDF canopy is chosen

[derekeder]

Sometimes getting the following error when running canonical example. This is most likely due to the invertedIndex function failing when no TF/IDF canopy is chosen

Traceback (most recent call last):
  File "test/canonical_test.py", line 123, in 
    blocked_data = dedupe.blocking.blockingIndex(data_d, blocker)
  File "/Users/derekeder/projects/open-city/deduplication/dedupe/dedupe/blocking.py", line 173, in blockingIndex
    blocker.invertIndex(data_d.iteritems())
  File "/Users/derekeder/projects/open-city/deduplication/dedupe/dedupe/blocking.py", line 74, in invertIndex
    num_docs = len(self.token_vector[field])
UnboundLocalError: local variable 'field' referenced before assignment