fix(seo): og:image and twitter:image are missing the img_path o…

…f the post (#754)

Resolves #754

_includes/head.html

@@ -26,14 +26,14 @@
     {% assign properties = 'og:image,twitter:image' | split: ',' %}
 
     {% for prop in properties %}
-      {% if site.img_cdn contains '//' %}
+      {% if site.img_cdn contains '://' %}
         <!-- `site.img_cdn` is a cross-origin URL -->
         {% capture target %}<meta property="{{ prop }}" content="{{ site.url }}{% endcapture %}
-        {% capture replacement %}<meta property="{{ prop }}" content="{{ site.img_cdn }}{% endcapture %}
+        {% capture replacement %}<meta property="{{ prop }}" content="{{ site.img_cdn }}{{ page.img_path }}{% endcapture %}
       {% else %}
         <!-- `site.img_cdn` is a local file path -->
         {% capture target %}<meta property="{{ prop }}" content="{{ site.url }}{{ site.baseurl }}{% endcapture %}
-        {% assign replacement = target | append: site.img_cdn %}
+        {% assign replacement = target | append: site.img_cdn | append: page.img_path %}
       {% endif %}
 
       {% assign seo_tags = seo_tags | replace: target, replacement %}