Removed XML::Lexer#buffering?

Instead of wrapping a predicate method around the ivar we'll just access it
directly. This reduces average lexing times in the big XML benchmark from 7,5
to ~7 seconds.

lib/oga/xml/lexer.rl

@@ -215,15 +215,6 @@ module Oga
         @buffer_start_position = position
       end
 
-      ##
-      # Returns `true` if we're currently buffering.
-      #
-      # @return [TrueClass|FalseClass]
-      #
-      def buffering?
-        return !!@buffer_start_position
-      end
-
       ##
       # Emits the current buffer if we have any. The current line number is
       # advanced based on the amount of newlines in the buffer.
@@ -538,7 +529,7 @@ module Oga
           # will otherwise take precedence over the other rules.
           any => {
             # First character, start buffering (unless we already are buffering).
-            start_buffer(ts) unless buffering?
+            start_buffer(ts) unless @buffer_start_position
 
             # EOF, emit the text buffer.
             if te == eof