More complex `rel` expression do not work

[Dav1dde]

More complex rel expression do currently not work with data-trunk:

<link data-trunk rel="alternate icon" type="image/x-icon" href="favicon.ico">

Error:

error from HTML pipeline

Caused by:
    unknown <link data-trunk .../> attr value `rel="alternate icon"`; please ensure the value is lowercase and is a supported asset type
Error: error from HTML pipeline

Caused by:
    unknown <link data-trunk .../> attr value `rel="alternate icon"`; please ensure the value is lowercase and is a supported asset type

Workaround (without hashing):

<link rel="alternate icon" type="image/x-icon" href="favicon.ico">
<link data-trunk rel="copy-file" href="favicon.ico">