Use a method for getting input in the XML lexer.

Instead of directly accessing the `data` instance variable the C/Java code now
uses the method `read_data`. This is part of one of the various steps required
to allow Oga to read data from IO like instances. It also means I can freely
change the name of the instance variable without also having to change the
C/Java code.

ext/java/org/liboga/xml/Lexer.rl

@@ -82,7 +82,7 @@ public class Lexer extends RubyObject
     public IRubyObject advance_native(ThreadContext context)
     {
         // Pull the data in from Ruby land.
-        RubyString rb_str = (RubyString) this.getInstanceVariable("@data");
+        RubyString rb_str = (RubyString) this.callMethod(context, "read_data");
         Encoding encoding = rb_str.getEncoding();
 
         byte[] data = rb_str.getBytes();