rssfilter produces invalid XML (unescaped &)

[dechamps]

If I run https://rssfilter.netlify.app/ on https://www.legislation.gov.uk/new/uksi/data.feed (regardless of filters), the resulting feed is not valid XML, preventing it from being used in RSS readers.

Since that feed changes over time, I have attached the original feed and its filtered version: https://github.com/bcongdon/rssfilter/files/15442737/uksi-rssfilter.zip

The errors are all variations of:

$ xmllint uksi-data-rssfilter.xml
uksi-data-rssfilter.xml:8: parser error : xmlParseEntityRef: no name
 Value="CORONERS, ENGLAND"></leg:heading><leg:heading Value="CORONERS, ENGLAND &
                                                                               ^
uksi-data-rssfilter.xml:8: parser error : xmlParseEntityRef: no name
CRIMINAL LAW, ENGLAND"></leg:heading><leg:heading Value="CRIMINAL LAW, ENGLAND &
                                                                               ^
uksi-data-rssfilter.xml:8: parser error : xmlParseEntityRef: no name
heading><leg:heading Value="CUSTOMS"></leg:heading><leg:heading Value="CUSTOMS &
                                                                               ^
…

And indeed, & characters are properly escaped in the input feed:

<leg:heading Value="CORONERS, ENGLAND &amp; WALES"/>

But they are not in the filtered feed:

<leg:heading Value="CORONERS, ENGLAND & WALES">