httr error with old token files

If `drop_acc` is used with a token generated while a <1.0.0 version of httr is installed, after updating httr, this cryptic error is thrown by httr:

`Error: is.request(y) is not TRUE`

This can be solved by re-generating the token file (or rolling back the httr version) but the error message is particularly unhelpful. It might be useful to add a warning about this.