Added assertion for updateLink

Added an assertion for string type. Since you could also pass objects
to `updateLink` (which would always result in false in the next if
statement) containing a link. This PR notifies creators of serializers
so they always use a string when handling links.
Maybe expand this error a bit, as when it just appears in the console it will be confusing. Think of your use case, what would've helped you debug the problem. Maybe something along the lines of, You pushed a record of type this.type, with a link for a relationship '+ this.key +' + but the link you pushed is not a string but + link. You probably need to fix your serializer to normalize the link correctly.