danbooru::http: factor out Cloudflare Polish bypassing.

* Factor out the Cloudflare Polish bypass code to a standalone feature.

* Add `http_downloader` method to the base source strategy. This is a
  HTTP client that should be used for downloading images or making
  requests to images. This client ensures that referrer spoofing and
  Cloudflare bypassing are performed.

This fixes a bug with the upload page reporting the polished filesize
instead of the original filesize when uploading ArtStation images.

app/logical/sources/strategies/base.rb

@@ -146,7 +146,7 @@ module Sources
 
       # Returns the size of the image resource without actually downloading the file.
       def remote_size
-        response = http.head(image_url)
+        response = http_downloader.head(image_url)
         return nil unless response.status == 200 && response.content_length.present?
 
         response.content_length.to_i
@@ -156,16 +156,23 @@ module Sources
       # Download the file at the given url, or at the main image url by default.
       def download_file!(download_url = image_url)
         raise DownloadError, "Download failed: couldn't find download url for #{url}" if download_url.blank?
-        response, file = http.download_media(download_url)
+        response, file = http_downloader.download_media(download_url)
         raise DownloadError, "Download failed: #{download_url} returned error #{response.status}" if response.status != 200
         file
       end
 
+      # A http client for API requests.
       def http
-        Danbooru::Http.headers(headers).public_only.timeout(30).max_size(Danbooru.config.max_file_size)
+        Danbooru::Http.new.public_only
       end
       memoize :http
 
+      # A http client for downloading files.
+      def http_downloader
+        http.timeout(30).max_size(Danbooru.config.max_file_size).use(:spoof_referrer).use(:unpolish_cloudflare)
+      end
+      memoize :http_downloader
+
       # The url to use for artist finding purposes. This will be stored in the
       # artist entry. Normally this will be the profile url.
       def normalize_for_artist_finder
@@ -292,8 +299,8 @@ module Sources
         to_h.to_json
       end
 
-      def http_exists?(url, headers = {})
-        http.headers(headers).head(url).status.success?
+      def http_exists?(url)
+        http_downloader.head(url).status.success?
       end
 
       # Convert commentary to dtext by stripping html tags. Sites can override

app/logical/sources/strategies/moebooru.rb

@@ -155,7 +155,7 @@ module Sources
 
           # the api_response wasn't available because it's a deleted post.
           elsif post_md5.present?
-            %w[jpg png gif].find { |ext| http_exists?("https://#{site_name}/image/#{post_md5}.#{ext}", headers) }
+            %w[jpg png gif].find { |ext| http_exists?("https://#{site_name}/image/#{post_md5}.#{ext}") }
 
           else
             nil