Fix #2460: Uploading pixiv user's background image throws error.

Fix exception when uploading these types of URLs:

* http://img12.pixiv.net/profile/rapattu/119950.jpg
* http://i2.pixiv.net/img130/profile/minono_aki/8733472.jpg
* http://i2.pixiv.net/img20/img/a-park/novel/3607898.jpg
* http://i1.pixiv.net/novel-cover-original/img/2016/11/03/20/10/58/7436075_f75af69f3eacd1656d3733c72aa959cf.jpg
* http://i1.pixiv.net/background/img/2016/05/17/12/05/48/2074388_d4ac52034f7ca0af3e083d59fde7e97f.jpg

Skip rewriting sources that don't have illust IDs to avoid this
exception. Also fix `tags` so that it doesn't blow up when fetching
source data for these types of sources.

app/logical/downloads/rewrite_strategies/pixiv.rb

@@ -8,9 +8,12 @@ module Downloads
       end
 
       def rewrite(url, headers, data = {})
-        if url =~ /https?:\/\/(?:\w+\.)?pixiv\.net/
+        if url =~ /\Ahttps?:\/\/(?:\w+\.)?pixiv\.net/
           url, headers = rewrite_headers(url, headers)
           url, headers = rewrite_cdn(url, headers)
+        end
+
+        if url =~ /\Ahttps?:\/\/(?:\w+\.)?pixiv\.net/ && source.illust_id_from_url
           url, headers = rewrite_html_pages(url, headers)
           url, headers = rewrite_thumbnails(url, headers)
           url, headers = rewrite_old_small_manga_pages(url, headers)

app/logical/sources/site.rb

@@ -7,7 +7,7 @@ module Sources
       :profile_url, :image_url, :tags, :artist_record, :unique_id, 
       :page_count, :file_url, :ugoira_frame_data, :ugoira_content_type, :image_urls,
       :has_artist_commentary?, :artist_commentary_title,
-      :artist_commentary_desc, :rewrite_thumbnails, :to => :strategy
+      :artist_commentary_desc, :rewrite_thumbnails, :illust_id_from_url, :to => :strategy
 
     def self.strategies
       [Strategies::PixivWhitecube, Strategies::Pixiv, Strategies::NicoSeiga, Strategies::DeviantArt, Strategies::Nijie, Strategies::Twitter, Strategies::Tumblr]

app/logical/sources/strategies/base.rb

@@ -74,6 +74,10 @@ module Sources
         [image_url]
       end
 
+      def tags
+        @tags || []
+      end
+
       # Should be set to a url for sites that prevent hotlinking, or left nil for sites that don't.
       def fake_referer
         nil

app/logical/sources/strategies/pixiv.rb

@@ -65,7 +65,8 @@ module Sources
       end
 
       def get
-        @illust_id = illust_id_from_url!
+        return unless illust_id_from_url
+        @illust_id = illust_id_from_url
         @metadata = get_metadata_from_papi(@illust_id)
 
         page = agent.get(URI.parse(normalized_url))