Merge implementation is not correct on SimpleCache

Merge method is not correctly implemented in SimpleCache.
Merge should work this way :

• if the key is not present, add the value
• If the key is present, apply the function between the existing key's value and the given value and replace the key with the merged result
• If the merge function returns null, remove the key

The case that should work as "putIfAbsent" does't work. Merge function is applied to null and the given value.