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/moebooru_test.rb

@@ -14,7 +14,7 @@ module Sources
       assert_equal(page_url, site.page_url) if page_url.present?
       assert_equal(tags.sort, site.tags.map(&:first).sort)
       assert_equal(profile_url.to_s, site.profile_url.to_s)
-      assert_equal(size, site.size)
+      assert_equal(size, site.remote_size)
       assert_nothing_raised { site.to_h }
     end