cast mailbox['quota'] to be integer.

see:  https://github.com/postfixadmin/postfixadmin/issues/342

model/MailboxHandler.php

@@ -271,6 +271,11 @@ class MailboxHandler extends PFAHandler {
         return true;
     }
 
+    protected function setmore(array $values) {
+        if(array_key_exists('quota', $this->values)) {
+            $this->values['quota'] = (int)$this->values['quota'];
+        }
+    }
 
     // Could perhaps also use _validate_local_part($new_value) { .... }
     public function set(array $values) {