Creating an RSS feed item with the inline description containing an `<img>` tag
with a `srcset` attribute pointing to an invalid URL like
`http:a<script>alert(1)</script>`, we can coerce the proxy handler into an error
condition where the invalid URL is returned unescaped and in full.
This results in JavaScript execution on the Miniflux instance as soon as the
user is convinced to open the broken image.
The image proxy buffered the whole image before sending it to the
browser. If the image is large and/or hosted on a slow server, this
caused a long delay before the user's browser could display anything.
