Issue: In JDK15, [URLConnection](https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/net/www/URLConnection.java) class overrode the getHeaderFields() methods to return properties, which previously returned an empty map from its super class.
Fix: Extract headers only when it's a HttpURLConnection, otherwise return an empty String. ------------- Commit messages: - Update copyright year - Add unit test - WebEngine refuses to load local "file:///" CSS stylesheets when using JDK 15 Changes: https://git.openjdk.java.net/jfx/pull/314/files Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=314&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8253696 Stats: 24 lines in 3 files changed: 15 ins; 0 del; 9 mod Patch: https://git.openjdk.java.net/jfx/pull/314.diff Fetch: git fetch https://git.openjdk.java.net/jfx pull/314/head:pull/314 PR: https://git.openjdk.java.net/jfx/pull/314
