gateway-ipns: CORS means we don't usually get an ETag *sigh*

plugins/gateway-ipns.js

@@ -102,7 +102,14 @@
                 
                 // add the X-Samizdat-* headers to the mix
                 init.headers['X-Samizdat-Method'] = 'gateway-ipns'
+                
+                // we will not have it most of the time, due to CORS rules:
+                // https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_response_header
                 init.headers['X-Samizdat-ETag'] = response.headers.get('ETag')
+                if (init.headers['X-Samizdat-ETag'] === null) {
+                    // far from perfect, but what are we going to do, eh?
+                    init.headers['X-Samizdat-ETag'] = response.headers.get('last-modified')
+                }
                 
                 // return the new response, using the Blob from the original one
                 return response