SVG author image not displaying on RSVP

RSVP post: https://iambismark.net/post/1530120533/

Appears in: https://2018.indieweb.org/#rsvps

Authorship algorithm as reported by sturdy-backbone: https://sturdy-backbone.glitch.me/test?url=https%3A%2F%2Fiambismark.net%2Fpost%2F1530120533%2F

```json
{
"type":["h-card"],
"properties":{
"photo":["https://iambismark.net/avatar.svg"],
"name":["Ryan Johnson"],
"url":["https://iambismark.net"],
"email":["mailto:ryan@iambismark.net"]
}
}
```

It looks like `img.php` supports SVG. Maybe 2018.indieweb.org doesn't follow step 6 of authorship to find author-page in `rel=author` to find the h-card? Then again, his homepage URL and name are found, so it must be finding the h-card.

Reproduce the RSVP entry at 2018.indieweb.org/#rsvps using the linked RSVP post and inspect the author-card data and image-loading path, including img.php. Compare the result with the authorship algorithm output; done means the SVG author image displays for this RSVP.

Written by the indexing model from the issue text.