Split prepare context middleware and fix unit test

lib/cartodb/middleware/context/parse-token-param.js

@@ -0,0 +1,47 @@
+module.exports = function parseTokenParamMiddleware () {
+    return function parseTokenParam (req, res, next) {
+        // jshint maxcomplexity:7
+        if (!req.params.token) {
+            return next();
+        }
+
+        var user = req.context.user;
+
+        // Token might match the following patterns:
+        // - {user}@{tpl_id}@{token}:{cache_buster}
+        var tksplit = req.params.token.split(':');
+
+        req.params.token = tksplit[0];
+
+        if ( tksplit.length > 1 ) {
+            req.params.cache_buster= tksplit[1];
+        }
+
+        tksplit = req.params.token.split('@');
+
+        if ( tksplit.length > 1 ) {
+            req.params.signer = tksplit.shift();
+
+            if ( ! req.params.signer ) {
+                req.params.signer = user;
+            } else if ( req.params.signer !== user ) {
+                var err = new Error(
+                    `Cannot use map signature of user "${req.params.signer}" on db of user "${user}"`
+                );
+                err.http_status = 403;
+                req.profiler.done('req2params');
+
+                return next(err);
+            }
+
+            // skip template hash
+            if (tksplit.length > 1) {
+                tksplit.shift();
+            }
+
+            req.params.token = tksplit.shift();
+        }
+
+        next();
+    };
+};