Fix for multiple values on "Link" header

[pabamato]

Sample header value with page as front page ( added by wp_shortlink_header() ): link →http://www.site.com/wp-json/; rel="http://api.w.org/", https://www.site.com/; rel=shortlink

Sample header value with posts as front page: Link →http://www.site.com/wp-json/; rel="https://api.w.org/"