rpc server: fix host filter for localhost on ipv6 #13848

Job	Run time
preflight / preflight	45s
preflight / ci-versions	1m 19s
test-linux-stable-int	9m 42s
test-linux-stable (1/3, parity-large)	38m 2s
test-linux-stable (1/3, parity-oldlinux)	36m 25s
test-linux-stable (2/3, parity-large)	29m 36s
test-linux-stable-no-try-runtime (1/2)	40m 42s
test-linux-stable (2/3, parity-oldlinux)	29m 43s
test-linux-stable-no-try-runtime (2/2)	32m 25s
test-linux-stable (3/3, parity-large)	29m 42s
test-linux-stable (3/3, parity-oldlinux)	28m 19s
test-linux-stable-runtime-benchmarks	29m 38s
All tests passed	1s
	5h 6m 19s

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