[push-api] Should receipt_endpoint be expose in PushSubscription of webpush api (#144)

PushSubsctiption has two parameter: endpoint and subscriptionId.

Webpush ietf draft have an additional parameter Receipt Endpoint. If it does not change, ReceiptEndpoint should be added. 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/144