[request] Ensure base_url() always has a trailing '/' (closes #3955093) Just as an example, cubicweb.ajax.js is clearly written under the assumption that BASE_URL/baseuri() - which simply contains req.base_url() - has a trailing /.

rql('SET X name "BoundaryConstraint" '
    'WHERE X is CWConstraintType, X name "BoundConstraint"')