Fix queries equalTo with null values (#406)

Author: zeliard91

src/Parse/ParseQuery.php

@@ -112,11 +112,7 @@ public function get($objectId, $useMasterKey = false)
      */
     public function equalTo($key, $value)
     {
-        if ($value === null) {
-            $this->doesNotExist($key);
-        } else {
-            $this->where[$key] = $value;
-        }
+        $this->where[$key] = $value;
 
         return $this;
     }

tests/Parse/ParseQueryTest.php

@@ -2207,11 +2207,20 @@ function ($i) {
                 return $obj;
             }
         );
+        $this->saveObjects(
+            2,
+            function ($i) {
+                $obj = ParseObject::create('TestObject');
+                $obj->set('num', null);
+
+                return $obj;
+            }
+        );
         $query = new ParseQuery('TestObject');
         $query->equalTo('num', null);
         $results = $query->find();
         $this->assertEquals(
-            0,
+            2,
             count($results),
             'Did not return correct number of objects.'
         );