Fails when multiple srcset values are present, and the first is remote

[paulroub]

Input HTML:

<source srcset="https://example.com/big.png 2x, https://example.com/small.png 1x" type="image/png" />

We're checking for "is the url remote?" before the check for "are there multiple srcsets?"

We then treat the whole string as one big remote, and get failures like:

Received a 404 for https://example.com/big.png%202x,%20https://example.com/small.png%201x

[paulroub]

PR #851 addresses this