RFR: 8253696: WebEngine refuses to load local "file:///" CSS stylesheets when using JDK 15

[Arun Joseph]

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