Add ability to delete session forcibly

[Daiki Kohara]

I now need to delete session by doing like this.

session.Options(sessions.Options{MaxAge: -1})
session.Clear() // or whatever changes session.written to true
session.Save()

I think it would be better if

• DeleteSession() method is added
  
  or
• Save() method takes a parameter to save session regardless of the session.written flag

I'm sorry if I'm missing something.