client/upload: send state of image's anonymous checkbox as boolean

Anonymous checkbox on images would not actually do anything, when the
global checkbox was unchecked. The value of anonymous becomes a node,
and it fails the anonymous === true check in save().
I don't understand why anonymous is treated differently from other
post parameters and supplied as an argument to save().
Keeping it the way it is, I guess...

client/js/views/post_upload_view.js

@@ -322,8 +322,8 @@ class PostUploadView extends events.EventTarget {
         }
 
         let anonymous = this._uploadAllAnonymous.checked;
-        if (!anonymous) {
-            anonymous = rowNode.querySelector(".anonymous input:checked");
+        if (!anonymous && rowNode.querySelector(".anonymous input:checked")) {
+            anonymous = true;
         }
         uploadable.anonymous = anonymous;