add convertStringExpiresInToNumber function for providers whose response's expires_in is a string instead of an integer(number)

[hotsixman]

Some providers(like Naver) do not adhere to OAuth 2.0 specs, its response's "expires_in" is not an integer but an string(not 3600 but "3600").
If a response(json) has "expires_in" and its type is string, this function will convert it to a number.

[panva]

#77 (comment)

There's already been PRs/issues open for this. expires_in in JSON responses is defined as a number, please reach out to your provider to get a fix on their end instead, or workaround the provider issue by instantiating a Response object with the original Response properties and the body type fixed.