danbooru::http: factor out referrer spoofing.

Factor out referrer spoofing so that it can be used outside of downloading
files. We also need to spoof the referrer when determining the remote
filesize of images on the uploads page.

test/unit/danbooru_http_test.rb

@@ -127,6 +127,15 @@ class DanbooruHttpTest < ActiveSupport::TestCase
       end
     end
 
+    context "spoof referrer feature" do
+      should "spoof the referer" do
+        response = Danbooru::Http.get("https://httpbin.org/anything")
+
+        assert_equal(200, response.status)
+        assert_equal("https://httpbin.org", response.parse.dig("headers", "Referer"))
+      end
+    end
+
     context "#download method" do
       should "download files" do
         response, file = Danbooru::Http.download_media("https://httpbin.org/bytes/1000")