[extension, getHeaders] Use a caseless comparison to get the response headers

The headers can be capitalized or not.  For instance « content-type » and «
Content-Type » should be treated equally, therefore we should make a caseless
comparison to get the requested headers.

extension/src/background/main.ts

@@ -102,7 +102,7 @@ function getHeader(
 ): string | null {
   if (headers === undefined) return null;
   for (let i = 0; i !== headers.length; i++) {
-    if (headers[i].name === name) {
+    if (headers[i].name.toLowerCase() === name.toLowerCase()) {
       let v = headers[i].value;
       if (v !== undefined) return v;
     }