Mark individual exercises as fetched

When someone fetches the next exercise (whichever it is) either
across all their active tracks or in a specific track, we post
back to the app so that we can show people their current status.

When someone fetches an individual exercise, we weren't posting back.
This fixes it for the individual fetch.

This does presume that the CLI passes along the API key, which it
originally was not doing. That's fixed in
exercism/cli#307
and it will be a part of the 2.2.6 release.

Author: kytrinyx

lib/xapi/exercism_io.rb

@@ -27,7 +27,7 @@ def self.code_for(key)
     end
 
     def self.fetch_for(key, language, slug)
-      post("/api/v1/iterations/#{language}/#{slug}/fetch", key)
+      post("/api/v1/iterations/#{language}/#{slug}/fetch", key) if !!key
     end
 
     def self.conn

v1/routes/exercises.rb

@@ -36,6 +36,11 @@ def homework
         # no need to authenticate for this one
         problem = config.find(language).find(slug)
         problem.validate or halt 404, { error: problem.error }.to_json
+        begin
+          Xapi::ExercismIO.fetch_for(params[:key], problem.track_id, problem.slug)
+        rescue
+          # don't fail just because we can't track it.
+        end
         pg :problems, locals: { problems: [problem] }
       end