Make getCharacterEncoding in UrlModuleSourceProvider protected

Allows subclasses to override the encoding used in case a local
javascript file is interpreted as being latin-1, when it should be
whatever the file is encoded with. Before openJDK 17.0.3 this was
automatically interpreted as utf-8.

src/org/mozilla/javascript/commonjs/module/provider/UrlModuleSourceProvider.java

@@ -161,7 +161,7 @@ private static Reader getReader(URLConnection urlConnection) throws IOException
                 urlConnection.getInputStream(), getCharacterEncoding(urlConnection));
     }
 
-    private static String getCharacterEncoding(URLConnection urlConnection) {
+    protected String getCharacterEncoding(URLConnection urlConnection) {
         final ParsedContentType pct = new ParsedContentType(urlConnection.getContentType());
         final String encoding = pct.getEncoding();
         if (encoding != null) {