Did you ever come to a solution? I attempted
https://api.twitch.tv/kraken/oauth2/token
?client_id=(INSERTID)
&client_secret=(INSERTSECRET)
&grant_type=refresh_token
&refresh_token=(INSERTTOKEN)
But Twitch returns:
“The parameter “grant_type” was malformed: value must be one of “authorization_code”, “password””