use ctime to fix image cache

app/helpers/application_helper.rb

@@ -126,10 +126,11 @@ module ApplicationHelper
   # 用户图像url，如果不存在的话，source为匿名用户，即默认使用匿名用户图像
   def url_to_avatar(source)
     if File.exist?(disk_filename(source.class, source.id))
+      ctime = File.ctime(disk_filename(source.class, source.id)).to_i
       if source.class.to_s == 'User'
-        File.join(relative_path, ["#{source.class}", "#{source.id}"])
+        File.join(relative_path, ["#{source.class}", "#{source.id}"]) + "?t=#{ctime}"
       else
-        File.join("images/avatars", ["#{source.class}", "#{source.id}"])
+        File.join("images/avatars", ["#{source.class}", "#{source.id}"]) + "?t=#{ctime}"
       end
     elsif source.class.to_s == 'User'
       str = source.user_extension.try(:gender).to_i == 0 ? "b" : "g"