Fixed compare_and_set on MutexAtomicReference.

The pure-Ruby version would return false if the lock could not be
obtained. This is distinctly different behavior from the C and Java
implementations which only return false if the values did not match.
This update brings parity to all implementations.

Author: jdantonio

lib/concurrent/atomic_reference/mutex_atomic.rb

@@ -42,14 +42,14 @@ def get_and_set(new_value)
 
     # @!macro atomic_reference_method_compare_and_set
     def _compare_and_set(old_value, new_value)
-      return false unless @mutex.try_lock
-      begin
-        return false unless @value.equal? old_value
-        @value = new_value
-      ensure
-        @mutex.unlock
+      @mutex.synchronize do
+        if @value == old_value
+          @value = new_value
+          true
+        else
+          false
+        end
       end
-      true
     end
   end
 end