ReactFiberHydrationWarning in react-reconciler: pretty attribute names

Print the extra attribute names array nicer, with a space after comma.
The default array toString was used implicitly before.

Author: sompylasar

packages/react-dom/src/__tests__/ReactMount-test.js

@@ -552,7 +552,7 @@ describe('ReactMount', () => {
         div,
       ),
     ).toWarnDev(
-      'Warning: Extra attributes from the server: data-ssr-extra-prop,data-ssr-extra-prop-2\n' +
+      'Warning: Extra attributes from the server: data-ssr-extra-prop, data-ssr-extra-prop-2\n' +
         '    in div (at **)',
     );
   });

packages/react-reconciler/src/ReactFiberHydrationWarning.js

@@ -624,7 +624,7 @@ function warnForExtraAttributes(attributeNames: Set<string>) {
     names.push(name);
   });
   // TODO: As we're here in the terminology of universal hydration, should we stop saying 'from the server'?
-  warning(false, 'Extra attributes from the server: %s', names);
+  warning(false, 'Extra attributes from the server: %s', names.join(', '));
 }
 
 function warnForDeletedHydratableInstance(