MXS-2468 Allow an http::Async object to be reset

maxutils/maxbase/include/maxbase/http.hh

@@ -175,7 +175,7 @@ public:
     };
 
     /**
-     * Defalt constructor creates an asynchronous operation whose status is ERROR.
+     * Defalt constructor creates an asynchronous operation whose status is READY.
      */
     Async();
 
@@ -204,6 +204,11 @@ public:
         return *this;
     }
 
+    /**
+     * Resets the instance so that it becomes as if it would have been default constructed.
+     */
+    void reset();
+
     /**
      * Return the status of the operation.
      *