Fix invariant/sanity check code.

The original code broke on an odd number of running threads.

Author: thedarkone

test/test_cache_loops.rb

@@ -299,7 +299,7 @@ def add_remove_to_zero(opts = {})
 
   def add_remove_to_zero_via_merge_pair(opts = {})
     code = <<-RUBY_EVAL
-      acc += (cache.merge_pair(key, key) {}) ? -1 : 1
+      acc += (cache.merge_pair(key, key) {}) ? 1 : -1
     RUBY_EVAL
     do_thread_loop(:add_remove_to_zero_via_merge_pair, code, {:loop_count => 5}.merge(opts)) do |result, cache, options, keys|
       assert_all_key_mappings_exist(cache, keys, false)