cannot unmarshal string into Go struct field TokenResponse.expires_in of type int64

[cluther]

Thank you for oauth2c! It's a useful tool that has helped me avoid writing one-off scripts on several occasions.

I was recently trying to use it to get a refresh token from login.microsoft.com, and encountered the following error.

failed to parse exchange response: json: cannot unmarshal string into Go struct field TokenResponse.expires_in of type int64

This happens because Microsoft has decided they're going to return expires_in as a JSON string instead of number. This seems to be a violation of the OAuth2 spec but they're doing it nonetheless.

I opened the pull request #119 with something that fixes it for me. Would you please take a look?

[mbilski]

Thank you! I've merged the PR and released it as v1.17.1