Handle DateTimes directly in the encoder

[ob-stripe]

r? @remi-stripe
cc @stripe/api-libraries

Handle DateTimes directly in the encoder, and update all DateTimes fields in options classes so that the conversion to long/string is no longer done internally (except for those that also have special values like "now").

[remi-stripe]

I'm assuming that trial_end is left. What would be the right approach to handle it?

[ob-stripe]

I'm assuming that trial_end is left. What would be the right approach to handle it?

Off the top of my head, we could define a special class DateTimeOrNow that could take care of its own encoding via its ToString method, or add a new case for the class in RequestStringBuilder. I'm not sure it's worth it if it's just used in one or two places though.