Set the default User-Agent sent for outgoing http requests.

* Add a `Danbooru.config.http_headers` config option.

* Refactor various places to use this option instead of setting the user
  agent manually.

app/logical/downloads/rewrite_strategies/base.rb

@@ -20,12 +20,8 @@ module Downloads
       end
 
     protected
-      def http_head_request(url, headers)
-        HTTParty.head(url, Danbooru.config.httparty_options.merge(headers: headers))
-      end
-
       def http_exists?(url, headers)
-        res = http_head_request(url, headers)
+        res = HTTParty.head(url, Danbooru.config.httparty_options.deep_merge(headers: headers))
         res.success?
       end
     end