Fix overwriting RediStore options in Delete() with Go 1.1

After deleting a session all subsequently created session cookies have  Expires and MaxAge set to wrong values:

 Expires=Thu, 01 Jan 1970 00:00:01 UTC; Max-Age=0

Reason for this seems to that the Go 1.1 compiler optimises out the intended copy in line 78 (session.Options = &(*s.Options)) which makes line 118 (options.MaxAge = -1) overwrite the original RediStore options instead of the private session copy.