Remove extra slashes from image meta url (#77)

- Update `default.html` to remove excess slashes

_layouts/default.html

@@ -26,7 +26,7 @@
     <meta property="og:title" content="{{ page.title | escape }}" />
     <meta property="og:description" content="{{ page.summary | escape }}"/>
     {% if page.image %}
-        {% assign thumbnailImage = site.canonical.url | append: '/' | append: page.image %}
+        {% assign thumbnailImage = page.image | prepend: '/' | replace : '//', '/' | prepend: site.canonical.url %}
     {% else %}
         {% assign thumbnailImage = site.canonical.url | append: '/assets/blog.png' %}
     {% endif %}