Avoid overwriting of the explicit ignore flag by the isCrowd flag

This is from cocodataset#465

Author: Atila Orhon

PythonAPI/pycocotools/cocoeval.py

@@ -106,7 +106,7 @@ def _toMask(anns, coco):
         # set ignore flag
         for gt in gts:
             gt['ignore'] = gt['ignore'] if 'ignore' in gt else 0
-            gt['ignore'] = 'iscrowd' in gt and gt['iscrowd']
+            gt['ignore'] = ('iscrowd' in gt and gt['iscrowd']) or gt['ignore']
             if p.iouType == 'keypoints':
                 gt['ignore'] = (gt['num_keypoints'] == 0) or gt['ignore']
         self._gts = defaultdict(list)       # gt for evaluation