Warn about ATS blocking

Sparkle/SUUpdater.m

@@ -142,6 +142,14 @@ -(void)checkIfConfiguredProperly {
     } else if (isMainBundle && !hasPublicDSAKey && !servingOverHttps) {
         SULog(@"WARNING: Serving updates over HTTP without signing them with a DSA key is deprecated and may not be possible in a future release. Please serve your updates over https, or sign them with a DSA key, or do both. See Sparkle's documentation for more information.");
     }
+
+#if __MAC_OS_X_VERSION_MAX_ALLOWED >= 101100
+    BOOL atsExceptionsExist = nil != [self.host objectForInfoDictionaryKey:@"NSAppTransportSecurity"];
+    if (!servingOverHttps && !atsExceptionsExist) {
+        [self showModalAlertText:@"Insecure feed URL is blocked in OS X 10.11"
+                 informativeText:[NSString stringWithFormat:@"You must change the feed URL (%@) to use HTTPS or disable App Transport Security.\n\nFor more information:\nhttp://sparkle-project.org/documentation/app-transport-security/", [feedURL absoluteString]]];
+    }
+#endif
 }