fixing issue with default_value for custom traits
fixing issue with default_value for custom traits

ChakriCherukuri/bqplot,ssunkara1/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,bloomberg/bqplot,dmadeka/bqplot,bloomberg/bqplot,ChakriCherukuri/bqplot,SylvainCorlay/bqplot,ssunkara1/bqplot,SylvainCorlay/bqplot,bloomberg/bqplot,dmadeka/bqplot