Cast translated displayname to string in CalDAV bday calendar

I just noticed an error in my previous PR that made actual sync fail with:

```xml
<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
  <s:exception>Sabre_DAV_Exception</s:exception>
  <s:message>Unknown property value type: object for property: {DAV:}displayname</s:message>
  <s:sabredav-version>1.7.6</s:sabredav-version>
</d:error>
```

@georgehrke