Fix SEO author bug

- If `twitter.username` is set and `author.twitter` is nil bad things happen.
- Close mmistakes#289

_includes/seo.html

@@ -25,7 +25,7 @@
   {% assign seo_description = seo_description | markdownify | strip_html | strip_newlines | escape_once %}
 {% endif %}
 
-{% assign seo_author = page.author | default: page.authors[0] | default: site.author %}
+{% assign seo_author = page.author | default: page.author[0] | default: site.author[0] %}
 {% if seo_author %}
   {% if seo_author.twitter %}
     {% assign seo_author_twitter = seo_author.twitter %}