fix typespec for date query to make keys optional

[mmmries]

👋 Hi!
Many thanks for building and maintaining stripity_stripe. It makes integrating with Stripe really nice. I noticed that when I pulled it into my project I started to get a dialyzer error because I was doing a query like Stripe.Event.list(%{created: %{gt: timestamp}, limit: 50, starting_after: cursor}, api: api_key)

The call above works just fine, but dialyzer is throwing an error because the typespec for the datequery specifies all 4 keys of gt, gte, lt, lte. Putting all of them right into the typespec will treat them as required keys. So I just changed the typespec to make each of those keys optional the way they are documented in the stripe api.

[coveralls]

Coverage remained the same at 82.505% when pulling c339c40 on mmmries:fix_typespec into 19ccd62 on code-corps:master.

[begedin]

Looks good. Thank you for submitting the fix and thank you for the great explanation on why it's needed. 👍