Fix db.collections not recognizing callback

Author: pluma

src/database.js

@@ -108,6 +108,10 @@ export default class Database {
   }
 
   collections (excludeSystem, cb) {
+    if (typeof excludeSystem === 'function') {
+      cb = excludeSystem
+      excludeSystem = undefined
+    }
     const {promise, callback} = this._connection.promisify(cb)
     this.listCollections(
       excludeSystem,