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/c/lexer.rl

@@ -70,12 +70,12 @@ void liboga_xml_lexer_callback_simple(VALUE self, const char *name)
 VALUE oga_xml_lexer_advance(VALUE self)
 {
     /* Pull the data in from Ruby land. */
-    VALUE data_ivar = rb_ivar_get(self, rb_intern("@data"));
+    VALUE data_block = rb_funcall(self, rb_intern("read_data"), 0);
 
     /* Make sure that all data passed back to Ruby has the proper encoding. */
-    rb_encoding *encoding = rb_enc_get(data_ivar);
+    rb_encoding *encoding = rb_enc_get(data_block);
 
-    char *data_str_val = StringValuePtr(data_ivar);
+    char *data_str_val = StringValuePtr(data_block);
 
     const char *p   = data_str_val;
     const char *pe  = data_str_val + strlen(data_str_val);

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();

lib/oga/xml/lexer.rb

@@ -64,6 +64,15 @@ module Oga
         @elements = []
       end
 
+      ##
+      # Returns the next block of data to lex.
+      #
+      # @return [String]
+      #
+      def read_data
+        return @data
+      end
+
       ##
       # Gathers all the tokens for the input and returns them as an Array.
       #