fix order of current/previous dependent root in REST SSE

[tersec]

#4119

nimbus-eth2/beacon_chain/consensus_object_pools/blockchain_dag.nim

Lines 1828 to 1836 in 4b3768c

	depRoot = withState(dag.headState): forkyState.proposer_dependent_root
	prevDepRoot = withState(dag.headState):
	forkyState.attester_dependent_root
	epochTransition = (finalizedHead != dag.finalizedHead)
	# TODO (cheatfate): Proper implementation required
	data = HeadChangeInfoObject.init(dag.head.slot, dag.head.root,
	getStateRoot(dag.headState),
	epochTransition, depRoot,
	prevDepRoot)

calls init with the attester_dependent_root, defined as depending root of get_previous_epoch, as the second dependent root parameter, while

nimbus-eth2/beacon_chain/consensus_object_pools/block_pools_types.nim

Lines 384 to 395 in 667c3c9

	func init*(t: typedesc[HeadChangeInfoObject], slot: Slot, blockRoot: Eth2Digest,
	stateRoot: Eth2Digest, epochTransition: bool,
	previousDutyDepRoot: Eth2Digest,
	currentDutyDepRoot: Eth2Digest): HeadChangeInfoObject =
	HeadChangeInfoObject(
	slot: slot,
	block_root: blockRoot,
	state_root: stateRoot,
	epoch_transition: epochTransition,
	previous_duty_dependent_root: previousDutyDepRoot,
	current_duty_dependent_root: currentDutyDepRoot
	)

expects it to be first.