FIX: improve last_modified date returned for avatars

instead of hard coding a date:

1. For optimized images use the upload date when on s3
2. For not-found use 10 minutes ago to match the expiry

app/controllers/user_avatars_controller.rb

@@ -124,7 +124,7 @@ class UserAvatarsController < ApplicationController
         optimized_path = Discourse.store.path_for(optimized)
         image = optimized_path if File.exists?(optimized_path)
       else
-        return proxy_avatar(Discourse.store.cdn_url(optimized.url))
+        return proxy_avatar(Discourse.store.cdn_url(optimized.url), upload.created_at)
       end
     end
 
@@ -141,7 +141,7 @@ class UserAvatarsController < ApplicationController
   end
 
   PROXY_PATH = Rails.root + "tmp/avatar_proxy"
-  def proxy_avatar(url)
+  def proxy_avatar(url, last_modified)
 
     if url[0..1] == "//"
       url = (SiteSetting.force_https ? "https:" : "http:") + url
@@ -163,8 +163,7 @@ class UserAvatarsController < ApplicationController
       FileUtils.mv tmp.path, path
     end
 
-    # putting a bogus date cause download is not retaining the data
-    response.headers["Last-Modified"] = DateTime.parse("1-1-2000").httpdate
+    response.headers["Last-Modified"] = last_modified.httpdate
     response.headers["Content-Length"] = File.size(path).to_s
     immutable_for(1.year)
     send_file path, disposition: nil
@@ -174,7 +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"] = DateTime.parse("1-1-2000").httpdate
+    response.headers["Last-Modified"] = 10.minutes.ago.httpdate
     response.headers["Content-Length"] = File.size(path).to_s
     send_file path, disposition: nil
   end

spec/requests/user_avatars_controller_spec.rb

@@ -79,7 +79,7 @@ describe UserAvatarsController do
 
       upload = Fabricate(:upload, url: "//test.s3.amazonaws.com/something")
 
-      Fabricate(:optimized_image,
+      optimized_image = Fabricate(:optimized_image,
         sha1: SecureRandom.hex << "A" * 8,
         upload: upload,
         width: 98,
@@ -98,6 +98,16 @@ describe UserAvatarsController do
 
       expect(response.body).to eq("image")
       expect(response.headers["Cache-Control"]).to eq('max-age=31556952, public, immutable')
+      expect(response.headers["Last-Modified"]).to eq(optimized_image.upload.created_at.httpdate)
+    end
+
+    it 'serves a correct last modified for render blank' do
+      freeze_time
+
+      get "/user_avatar/default/xxx/51/777.png"
+
+      expect(response.status).to eq(200)
+      expect(response.headers["Last-Modified"]).to eq(10.minutes.ago.httpdate)
     end
 
     it 'serves image even if size missing and its in local mode' do