Only close image if it wasn't already open

Fixes #1256

wagtail/wagtailimages/models.py

@@ -95,13 +95,19 @@ class AbstractImage(models.Model, TagSearchable):
             # these from IOErrors elsewhere in the process
             raise SourceImageIOError(text_type(e))
 
-        image_file.open('rb')
+        # Open file if it is closed
+        close_file = False
+        if image_file.closed:
+            image_file.open('rb')
+            close_file = True
+
         image_file.seek(0)
 
         try:
             yield WillowImage.open(image_file)
         finally:
-            image_file.close()
+            if close_file:
+                image_file.close()
 
     def get_rect(self):
         return Rect(0, 0, self.width, self.height)

wagtail/wagtailimages/tests/test_models.py

@@ -238,7 +238,6 @@ class TestGetWillowImage(TestCase):
 
         self.assertTrue(self.image.file.closed)
 
-    @unittest.expectedFailure
     def test_doesnt_close_open_image(self):
         # This tests that when the image file is already open, get_willow_image doesn't close it (#1256)
         self.image.file.open('rb')