Set file stream curstor to zero position after running validators (#218)

The `BaseSizeValidator` read the file stream to create an in memory Pillow Image object. In reading the file, the file streams cursor was set to the end. Therefore, the validator left the file in a different state than receiving it. This patch sets the cursor back to zero after reading the file.

stdimage/validators.py

@@ -28,8 +28,9 @@ def __call__(self, value):
     def clean(value):
         value.seek(0)
         stream = BytesIO(value.read())
-        img = Image.open(stream)
-        return img.size
+        size = Image.open(stream).size
+        value.seek(0)
+        return size
 
 
 class MaxSizeValidator(BaseSizeValidator):

tests/test_models.py

@@ -227,15 +227,17 @@ def test_render_variations_overwrite(self, db, image_upload_file):
 
 class TestValidators(TestStdImage):
     def test_max_size_validator(self, admin_client):
-        admin_client.post('/admin/tests/maxsizemodel/add/', {
+        response = admin_client.post('/admin/tests/maxsizemodel/add/', {
             'image': self.fixtures['600x400.jpg'],
         })
+        assert 'too large' in response.context['adminform'].form.errors['image'][0]
         assert not os.path.exists(os.path.join(IMG_DIR, '800x600.jpg'))
 
     def test_min_size_validator(self, admin_client):
-        admin_client.post('/admin/tests/minsizemodel/add/', {
+        response = admin_client.post('/admin/tests/minsizemodel/add/', {
             'image': self.fixtures['100.gif'],
         })
+        assert 'too small' in response.context['adminform'].form.errors['image'][0]
         assert not os.path.exists(os.path.join(IMG_DIR, '100.gif'))