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.

test/unit/sources/pixiv_test.rb

@@ -97,6 +97,10 @@ module Sources
           assert_equal("uroobnad", @site.artist_name)
         end
 
+        should "get the remote image size" do
+          assert_equal(854_653, @site.remote_size)
+        end
+
         should "get the full size image url" do
           assert_equal("https://i.pximg.net/img-original/img/2017/11/21/05/12/37/65981735_p0.jpg", @site.image_url)
         end