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 - davidsantiago/quoin#4