Revert "Reset @@parser in between specs"

This reverts commit b562bed.

Closes #28.

lib/multi_xml.rb

@@ -57,7 +57,7 @@ class ParseError < StandardError; end
   class << self
     # Get the current parser class.
     def parser
-      return @@parser unless @@parser.nil?
+      return @@parser if defined?(@@parser)
       self.parser = self.default_parser
       @@parser
     end

spec/multi_xml_spec.rb

@@ -5,10 +5,6 @@ class MockDecoder; end
 
 describe "MultiXml" do
   context "Parsers" do
-    before do
-      MultiXml.send :class_variable_set, :@@parser, nil
-    end
-
     it "should pick a default parser" do
       MultiXml.parser.should be_kind_of(Module)
       MultiXml.parser.should respond_to(:parse)