at_period_end=truein the request. That's fine. RFC 7231 section 4.3.5 says that a DELETE request is allowed to have a body but it "has no defined semantics".
Unfortunately, App Engine's urlfetch service silently removes the body from all outgoing DELETE requests. Trying to cancel a Stripe subscription in App Engine always does it immediately, even if you asked to cancel at the end of the billing period. Google's known about the problem since 2008, but never fixed it. The glitch impacts many APIs other than Stripe.
Fortunately, you can use App Engine's sockets API to construct a workaround. In Go, you build an HTTP client whose transport uses the socket API to make outbound network connections. That uses Go's built in support for HTTP DELETE requests and avoids the bugs in App Engine's urlfetch service.