Add Access-Control-Expose-Headers field constant:

This response header is described both in [CORS](https://www.w3.org/TR/cors/)
and [Fetch](https://fetch.spec.whatwg.org/) specifications.

tools/field.txt

@@ -13,6 +13,7 @@ Access-Control-Allow-Credentials
 Access-Control-Allow-Headers
 Access-Control-Allow-Methods
 Access-Control-Allow-Origin
+Access-Control-Expose-Headers
 Access-Control-Max-Age
 Access-Control-Request-Headers
 Access-Control-Request-Method