Doesn't always properly parse expires_at

[jeffutter]

I'm dealing with an API that returns the authorize request as such:

{ "access_token": "k8sbPR4is2C7ipTYgEbi8fe470mp", "refresh_token": "dQJiREMfaHhDBoGohIj7JEpIOYYk9Jif", "expires_in": "599" }

With expires_in as a string. With the existing code, here: https://github.com/scrogson/oauth2/blob/master/lib/oauth2/access_token.ex#L137 it would parse that into the integer 599, which is still not the correct value for expires_at. In this case, it would have to call expires_at again with the integer value to get the timestamp.

I'm not sure if having it call expires_at(int) at the end of the string version would break anything else or not.