Fixes a bug when detecting changes on a null value

If the new value was equal to `null`, it was then always registered
as a `Removal` instead of possibly a `Addition` or `Modification`...
or not a change (in the case that the old value was also `null`).

This is due to the fact that `isset($a)` returns false even if the
key is set but its value is `null`.

src/Totem/Set.php

@@ -139,7 +139,7 @@ private function compute(AbstractSnapshot $old, AbstractSnapshot $new)
         $this->changes = [];
 
         foreach ($old->getDataKeys() as $key) {
-            if (!isset($new[$key])) {
+            if (!in_array($key, $new->getDataKeys())) {
                 $this->changes[$key] = new Removal($old[$key] instanceof AbstractSnapshot ? $old[$key]->getRawData() : $old[$key]);
                 continue;
             }