Support primitives in an anyOf from the parameters coercer

[brandur-stripe]

I was running stripe-ruby's test suite against a recent version of the
OpenAPI, and noticed that unfortunately, recent updates have broken it.
In particular, the new tiers feature under plan creation now has a
primitive type nested in an anyOf, which is something that we've never
had to handle before:

up_to:
  anyOf:
  - enum:
    - inf
    type: string
  - type: integer

stripe-ruby tries to run this against it:

plan = Stripe::Plan.create(
  ...
  tiers: [{ up_to: 100, amount: 1000 }, { up_to: "inf", amount: 2000 }]
)

What's supposed to happen is that "100" is coerced to an integer, but it's not
because the integer possibility is nested in an anyOf as seen above.

This patch addresses the problem by allowing the coercer to recurse into an
anyOf to try coercion. It's able to theoretically handle any number of
anyOf possibilities, and will dutifully try coercing each one until it finds
a coercable type (or it doesn't, which is okay).

I've run stripe-ruby's test suite against a version of stripe-mock with
this patch in it to verify that it works.

r? @tmaxwell-stripe (This one should be a little simpler to review than the other PR.)
cc @stripe/api-libraries

[tmaxwell-stripe]

LGTM

[brandur-stripe]

Thanks!

[alexander-stripe]

Ah nice!