Migrate sql base toString method not accessible by subclasses

Below is the current __toString method of SQLBase, when calling this however from a subclass, we can't access this result, since we return the protected variable $query instead of the public query() method.

/**
   * Prints the query string when the object is used as a string.
   *
   * @return string
   *   The query string.
   */
  public function __toString() {
    return (string) $this->query;
  }

Patch is simpler of issue description ;-)