[ELPA/oauth2] Sometimes oauth2-refresh-access does not work correctly

[Aleksandr Vityazev]

Hello, thank you for maintaining such a wonderful package!

While working on my own package, I found that oauth2 cannot properly
handle the situation where the refresh_token changes every time I
refresh the access. If I use plsore to store the token, I can only
update the token once, and after that I have to delete plstore to get
new access.

Can you add a refresh_token change check to the 'oauth2-refresh-access'
function? And if there are changes, then update the refresh-token and
access-response fields as well as the access-token.

-- 
Best regards,
Aleksandr Vityazev