When sending an ARI response, write the Retry-After header before
writing the JSON response body. This is necessary because
http.ResponseWriter implicitly calls WriteHeader whenever Write is
called, flushing all headers to the network and preventing any
additional headers from being written. Unfortunately, the unittests use
httptest.ResponseRecorder, which doesn't seem to enforce this invariant
(it's happy to report headers which were written after the body). Add a
header check to the integration tests, to make up for this deficiency.
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