Incorrect handling of resumable upload responses without a range header

[coryan]

The client library treats 308 responses as an unrecoverable error:

google-cloud-cpp/google/cloud/storage/internal/retry_resumable_upload_session.cc

Line 76 in 49b5cc2

std::uint64_t new_next_byte = session_->next_expected_byte();

which can be triggered from here:

google-cloud-cpp/google/cloud/storage/internal/resumable_upload_session.cc

Lines 50 to 57 in 49b5cc2

	auto r = response.headers.find("range");
	if (r == response.headers.end()) {
	std::ostringstream os;
	os << __func__ << "() missing range header in resumable upload response"
	<< ", response=" << response;
	result.annotations += " " + std::move(os).str();
	return result;
	}

However the spec allows for an empty range header:

If Cloud Storage has not yet persisted any bytes, the 308 response does not have a Range header. In this case, you should start your upload from the beginning.

https://cloud.google.com/storage/docs/performing-resumable-uploads