customCSS URL rewriting exemption-check is broken

[rjgotten]

URLs rewritten inside customCSS have an exemption list, but it is incomplete and in part broken.

Current exemption logic is:

if (
  !src_path.startsWith('http') &&
  !src_path.startsWith('//') &&
  !src_path.startsWith('data:')
) {
  // ...
}

This has two problems:

• It fails to rewrite relative urls such as url("http_foo/bar.jpg") which are most definitely relative, because the starting check for HTTP is overly naive.
• It mistakingly rewrites domain-relative paths such as url("/foo/bar.jpg"), because it fails to detect them outright.

You can cover all the necessary cases with a simple regex, afaict:

let regNoRewrite = /^(?:https?\:|data\:|\/)/i;
if ( !regNoRewrite.test(src_path)) {
  // ...
}

[jgerigmeyer]

Good catch! As I said over on #265: a PR (with tests) would be welcome; otherwise I'll try to get to this in the next week.

[jgerigmeyer]

@rjgotten Would you be willing to try against the master branch and see if this is adequately fixed? Thanks!