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.

Added test for links with type other than string
Removed a duplicate line.
Added test case for push link with null value
Merge pull request #2354 from steffenbrem/update-links-assert-fix

Added assertion for updateLink