Fix a warning

Author: JasonBarnabe

lib/qbxml/hash.rb

@@ -28,7 +28,7 @@ def self.from_hash(hash, opts = {}, &block)
   end
 
   def to_xml(opts = {})
-    hash = self.class.to_xml(self, opts)
+    self.class.to_xml(self, opts)
   end
 
   def self.to_xml(hash, opts = {})