http: Fix interpretation of non-numerical parameters.

Fixes a regression introduced in
593cb7be.

* src/cuirass/http.scm (request-parameters): Fix fallback case in
'match' form.  Previously it would return a procedure in this case, as
returned by (const param), leading to a failure down the road in
'sqlite-bind-arguments' as could be seen by running tests/http.scm.