Mark session as written when updating options

[andrewfrench]

This PR changes the behavior of the Options() method.

Currently, updating session options does set the written flag to true. This causes an issue when expiring/destroying a session, for example, because changes to session options will not be updated unless a key/value update has been made. If a developer wants to expire a session, they must set MaxAge to -1 and then make a trivial call to a method like Set() to ensure the options change will persist:

// Destroy session
session := sessions.Default(c)
session.Options(sessions.Options{
    MaxAge: -1,
})
session.Set("deleteMe", true)
_ = session.Save()

Documentation from the session stores, like this snippet from the memstore source, indicate that setting MaxAge to -1 should immediately expire a session:

https://github.com/quasoft/memstore/blob/2bce066d2b0b885eab067015d658dfd8ec1880e0/memstore.go#L103

To make this library more usable, I've updated the Options() method to mark the session as written when session options have been updated, saving the additional call:

// Destroy session
session := sessions.Default(c)
session.Option(sessions.Options{
    MaxAge: -1,
})
_ = session.Save()

Passing tests, including a case to test this behavior, can be found here.

[kpritam]

@appleboy do you think this can be merged? Any suggestion from your side on this?