Do not clobber v:this_session during session save

[inkarkat]

Hello Peter, haven't heard from each other in quite a while. I've been using your plugin(s) happily :-) I hope you're well and you still maintain your plugins, because here's a tiny fix:

When saving a session, the plugin uses :mksession with a temp file, whose name is generated via tempname(). If no session so far existed, Vim will derive a session name (and put this into v:this_session) from that temp file name, e.g. /tmp/v3CCl7f/20 will turn into v:this_session = "20". Now, when another (unnamed) session is saved, the plugin will pick up v:this_session and use "20" instead of "default", which is irritating and unexpected. Easy fix: Save and restore the value of v:this_session.

[romgrk]

Hey, I'm trying to create a community maintained version of this plugin due to the disparition of the maintainer, if you're interested you can open this PR at https://github.com/romgrk/vim-session.

https://github.com/xolox/vim-session/issues/185