Ignore protocol relative '//' urls.

plugins/octopress_filters.rb

@@ -78,8 +78,8 @@ module OctopressLiquidFilters
   # Replaces relative urls with full urls
   def expand_urls(input, url='')
     url ||= '/'
-    input.gsub /(\s+(href|src)\s*=\s*["|']{1})(\/[^\"'>]*)/ do
-      $1+url+$3
+    input.gsub /(\s+(href|src)\s*=\s*["|']{1})(\/[^\/\"'>]*)(["|']{1})/ do
+      $1 + expand_url($3, url) + $4
     end
   end