Add test for str dtype len discovery with converters.
Add test for str dtype len discovery with converters.

nrows gt chunksize.

pdebuyl/numpy,jakirkham/numpy,charris/numpy,endolith/numpy,endolith/numpy,jakirkham/numpy,pdebuyl/numpy,endolith/numpy,anntzer/numpy,mhvk/numpy,charris/numpy,mattip/numpy,charris/numpy,mhvk/numpy,anntzer/numpy,seberg/numpy,jakirkham/numpy,endolith/numpy,mattip/numpy,pdebuyl/numpy,mattip/numpy,mhvk/numpy,pdebuyl/numpy,rgommers/numpy,numpy/numpy,rgommers/numpy,rgommers/numpy,seberg/numpy,seberg/numpy,charris/numpy,rgommers/numpy,anntzer/numpy,seberg/numpy,anntzer/numpy,numpy/numpy,mhvk/numpy,jakirkham/numpy,numpy/numpy,mattip/numpy,jakirkham/numpy,mhvk/numpy,numpy/numpy