Fix startedAt in progress API

If no progress had been set before, setting `startedAt` did not work and
it would always been set to `finishedAt` or `Date.now()`. Settings this
if any progress had already been recorded did work.

This fixes the problem so that setting `startedAt` it properly works in
both cases.

server/objects/user/MediaProgress.js

@@ -68,7 +68,7 @@ class MediaProgress {
       this.finishedAt = progress.finishedAt || Date.now()
       this.progress = 1
     }
-    this.startedAt = this.finishedAt || Date.now()
+    this.startedAt = progress.startedAt || this.finishedAt || Date.now()
   }
 
   update(payload) {