Uncaught error in attributes when not string

[alexloi]

Reproduce like so:
This works:
!=CDN('/images/brand/logo-hi-def.png', {'class': 'sh-logo'})

This fails without a clear error message:
!=CDN('/images/brand/logo-hi-def.png', {class: 'sh-logo'})

I guess this should still fail, I'll add checking if the attribute is a string and fail with a readable error message cause this specific use case might happen a lot (simple typo).