uploads: fix incorrect remote sizes on pixiv uploads.

Bug: the uploads page showed a remote size of 146 bytes for Pixiv uploads.

Cause: we didn't spoof the Referer header when making the HEAD request
for the image, causing Pixiv to return a 403 error.

Also fix the case where the Content-Length header is absent.

app/logical/sources/strategies/base.rb

@@ -145,10 +145,13 @@ module Sources
       end
 
       # Returns the size of the image resource without actually downloading the file.
-      def size
-        http.head(image_url).content_length.to_i
+      def remote_size
+        response = http.head(image_url)
+        return nil unless response.status == 200 && response.content_length.present?
+
+        response.content_length.to_i
       end
-      memoize :size
+      memoize :remote_size
 
       # Download the file at the given url, or at the main image url by default.
       def download_file!(download_url = image_url)
@@ -159,7 +162,7 @@ module Sources
       end
 
       def http
-        Danbooru::Http.public_only.timeout(30).max_size(Danbooru.config.max_file_size)
+        Danbooru::Http.headers(headers).public_only.timeout(30).max_size(Danbooru.config.max_file_size)
       end
       memoize :http
 

app/logical/upload_service/controller_helper.rb

@@ -8,7 +8,7 @@ class UploadService
         UploadPreprocessorDelayedStartJob.perform_later(url, ref, CurrentUser.user)
 
         strategy = Sources::Strategies.find(url, ref)
-        remote_size = strategy.size
+        remote_size = strategy.remote_size
 
         return [upload, remote_size]
       end

app/views/uploads/_image.html.erb

@@ -1,7 +1,7 @@
 <% if params[:url] %>
   <p id="upload-image-metadata">
     <strong>Size</strong>
-    <% if @remote_size %>
+    <% if @remote_size.present? %>
       <span id="upload-image-metadata-filesize"><%= number_to_human_size(@remote_size) %></span>
     <% end %>
     <span id="upload-image-metadata-resolution"></span>