Ensure that keywords take precedence over strings

When there is a keyword and a string in the same map, the keyword is supposed to take precedence. However, if the value matching the keyword is nil, then Quoin's `get-named`'s use of or skips over the keyword's value. Using contains-named? fixes this.

I've submitted the patch to Quoin directly too - https://github.com/davidsantiago/quoin/pull/4