Fixed approvalPrompt, viewPrivate and write in getRequestAccessUrl

According to docs,

> approval_prompt:	string optional 
> ‘force’ or ‘auto’, use ‘force’ to always show the authorization prompt even if the user has already authorized the current application, default is ‘auto’
> scope:	string optional 
> comma delimited string of ‘view_private’ and/or ‘write’, leave blank for read-only permissions.