fix: Unbreak tests when using Nokogiri 1.10.2
fix: Unbreak tests when using Nokogiri 1.10.2

Nokogiri 1.10.2 introduced a patch to the vendored libxml2 lib that
slightly changed the way server-side include markup is escaped, which
broke our tests:

https://github.com/sparklemotion/nokogiri/commit/4852e43cb6039e26d8c51af78621e539cbf46c5d

This only affects the rare case where a sanitized node is serialized
manually using Nokogiri's `to_html` method, so is unlikely to impact
most users. When Sanitize itself serializes nodes to HTML it always uses
Nokogumbo's standard-compliant sanitizer, whose output hasn't changed.

rgrove/sanitize,rgrove/sanitize