minor: OrderedHash hash algorithm fix

lib/mongo/util/ordered_hash.rb

@@ -120,7 +120,7 @@ class OrderedHash < Hash
         code = 37 * code + key.hash
         code = 37 * code + value.hash
       end
-      code
+      code & 0x7fffffff
     end
 
     def eql?(o)

test/test_ordered_hash.rb

@@ -19,6 +19,12 @@ class OrderedHashTest < Test::Unit::TestCase
     assert_equal a, b
   end
 
+  def test_hash_code
+    o = OrderedHash.new
+    o['number'] = 50
+    assert o.hash
+  end
+
   def test_empty
     assert_equal [], OrderedHash.new.keys
   end