use Array#permutation if it exists

otherwise create a Array#permutation method with the permutation gem on
the fly

tests/test_json.rb

@@ -9,6 +9,20 @@
 end
 require 'stringio'
 
+unless Array.method_defined?(:permutation)
+  begin
+    require 'enumerator'
+    require 'permutation'
+    class Array
+      def permutation
+        Permutation.for(self).to_enum.map { |x| x.project }
+      end
+    end
+  rescue LoadError
+    warn "Skipping permutation tests."
+  end
+end
+
 class TC_JSON < Test::Unit::TestCase
   include JSON
 
@@ -94,30 +108,24 @@ def test_parse_simple_objects
     assert_equal({ "a" => 0.23 }, parse('  {  "a"  :  0.23  }  '))
   end
 
-  begin
-    require 'permutation'
+  if Array.method_defined?(:permutation)
     def test_parse_more_complex_arrays
       a = [ nil, false, true, "foßbar", [ "n€st€d", true ], { "nested" => true, "n€ßt€ð2" => {} }]
-      perms = Permutation.for a
-      perms.each do |perm|
-        orig_ary = perm.project
-        json = pretty_generate(orig_ary)
-        assert_equal orig_ary, parse(json)
+      a.permutation.each do |perm|
+        json = pretty_generate(perm)
+        assert_equal perm, parse(json)
       end
     end
 
     def test_parse_complex_objects
       a = [ nil, false, true, "foßbar", [ "n€st€d", true ], { "nested" => true, "n€ßt€ð2" => {} }]
-      perms = Permutation.for a
-      perms.each do |perm|
+      a.permutation.each do |perm|
         s = "a"
-        orig_obj = perm.project.inject({}) { |h, x| h[s.dup] = x; s = s.succ; h }
+        orig_obj = perm.inject({}) { |h, x| h[s.dup] = x; s = s.succ; h }
         json = pretty_generate(orig_obj)
         assert_equal orig_obj, parse(json)
       end
     end
-  rescue LoadError
-    warn "Skipping permutation tests."
   end
 
   def test_parse_arrays