BUG : fix handling of flierprop by boxplot

 - moved logic to parse sym from bxp -> boxplot
 - restored documented behavior with `sym=''`
 - change default value of sym from 'b+' -> None
   - should not be visible from outside as default behavior remains
     the same

closes #3459
Merge pull request #3533 from tacaswell/fix_flierprop

BUG : fix handling of flierprop by boxplot