title: get_the_post_thumbnail_url gives weird results
body: <p>This is for an RSS feed</p>
  <p>EDIT: This was a question originally about get_the_post_thumbnail_url, but ultimately the output of this function was being overwritten by a filter.</p>
  <p>However amarinediary did provide a correct answer, below.</p>
  <p>Also, if anyone has difficulties with gmail not showing images in an email, know that prepending the img src with https: solved the problem for me.</p>
  <p>Thanks, Richard</p>