Do not use a default encoding when parsing XML documents.

This change fixes the tests in #2080, but introduces more errors.  The errors
are mostly unexpected null encoding when parsing an HTML document.

ext/java/nokogiri/internals/XmlDomParserContext.java

@@ -93,7 +93,7 @@ public XmlDomParserContext(Ruby runtime, IRubyObject options) {
     public XmlDomParserContext(Ruby runtime, IRubyObject encoding, IRubyObject options) {
         super(runtime);
         this.options = new ParserContext.Options(RubyFixnum.fix2long(options));
-        java_encoding = NokogiriHelpers.getValidEncoding(encoding);
+        java_encoding = NokogiriHelpers.getValidEncodingOrNull(encoding);
         ruby_encoding = encoding;
         initErrorHandler();
         initParser(runtime);