Add test to check the headers exist while instantiating a map

lib/api/middlewares/client-header.js

@@ -0,0 +1,11 @@
+'use strict';
+
+module.exports = function clientHeader () {
+    return function clientHeaderMiddleware (req, res, next) {
+        const { client } = req.query;
+
+        res.set('Carto-Client', client);
+
+        return next();
+    };
+};