FIX: set old last modified date for invalid avatars

In some cases Akami was holding tight to these invalid avatars,
to avoid this happening we explain the avatar image is ancient
then when a new upload is added it automatically is older than
this.

app/controllers/user_avatars_controller.rb

@@ -173,7 +173,7 @@ class UserAvatarsController < ApplicationController
   def render_blank
     path = Rails.root + "public/images/avatar.png"
     expires_in 10.minutes, public: true
-    response.headers["Last-Modified"] = 10.minutes.ago.httpdate
+    response.headers["Last-Modified"] = Time.new('1990-01-01').httpdate
     response.headers["Content-Length"] = File.size(path).to_s
     send_file path, disposition: nil
   end