Avoid AWS_DEFAULT_ACL warning when overridden as a class variable

storages/backends/s3boto3.py

@@ -277,7 +277,7 @@ def __init__(self, acl=None, bucket=None, **settings):
             self.config = Config(**kwargs)
 
         # warn about upcoming change in default AWS_DEFAULT_ACL setting
-        if not hasattr(django_settings, 'AWS_DEFAULT_ACL'):
+        if not hasattr(django_settings, 'AWS_DEFAULT_ACL') and self.default_acl == 'public-read':
             warnings.warn(
                 "The default behavior of S3Boto3Storage is insecure and will change "
                 "in django-storages 2.0. By default files and new buckets are saved "

tests/test_s3boto3.py

@@ -579,3 +579,25 @@ def test_deprecated_bucket(self):
             "argument will be removed in version 2.0."
         )
         assert str(w[-1].message) == message
+
+    def test_deprecated_default_acl(self):
+        with warnings.catch_warnings(record=True) as w:
+            s3boto3.S3Boto3Storage()
+        assert len(w) == 1
+        message = (
+            "The default behavior of S3Boto3Storage is insecure and will change "
+            "in django-storages 2.0. By default files and new buckets are saved "
+            "with an ACL of 'public-read' (globally publicly readable). Version 2.0 will "
+            "default to using the bucket's ACL. To opt into the new behavior set "
+            "AWS_DEFAULT_ACL = None, otherwise to silence this warning explicitly "
+            "set AWS_DEFAULT_ACL."
+        )
+        assert str(w[-1].message) == message
+
+    def test_deprecated_default_acl_override_class_variable(self):
+        class MyStorage(s3boto3.S3Boto3Storage):
+            default_acl = "private"
+
+        with warnings.catch_warnings(record=True) as w:
+            MyStorage()
+        assert len(w) == 0