Next 9.5 update (fall 2020)

[eric-burel]

• The call to detect static export is probably wrong in newer version of Next, because now headers is defined as an empty object.

We should change to:

   !!(
     appContext &&
     appContext.ctx &&
-    appContext.ctx.req &&
-    !appContext.ctx.req.headers
+    appContext.ctx.res &&
+    !appContext.ctx.res.writeHead
   );

• We don't need to polyfill fetch anymore, it is included in Next 9.4
• Others?