fix: `PromisePoolCluster.of` returns `PromisePoolCluster` instead of `PoolNamespace` #877

Job	Run time
mysql:5.7 - SSL=0 Compression=0	6h 0m 12s
mysql:5.7 - SSL=1 Compression=0	6h 0m 13s
mysql:5.7 - SSL=0 Compression=1	6h 0m 13s
mysql:5.7 - SSL=1 Compression=1	6h 0m 15s
mysql:8.0.18 - SSL=0 Compression=0	1m 12s
mysql:8.0.18 - SSL=1 Compression=0	1m 9s
mysql:8.0.18 - SSL=0 Compression=1	1m 7s
mysql:8.0.18 - SSL=1 Compression=1	1m 15s
mysql:8.0.22 - SSL=0 Compression=0	1m 13s
mysql:8.0.22 - SSL=1 Compression=0	1m 10s
mysql:8.0.22 - SSL=0 Compression=1	1m 8s
mysql:8.0.22 - SSL=1 Compression=1	1m 13s
mysql:8.0.33 - SSL=0 Compression=0	1m 9s
mysql:8.0.33 - SSL=1 Compression=0	1m 17s
mysql:8.0.33 - SSL=0 Compression=1	1m 10s
mysql:8.0.33 - SSL=1 Compression=1	1m 18s
mysql:9.0.1 - SSL=0 Compression=0	1m 10s
mysql:9.0.1 - SSL=1 Compression=0	1m 18s
mysql:9.0.1 - SSL=0 Compression=1	1m 12s
mysql:9.0.1 - SSL=1 Compression=1	1m 19s
mysql:latest - SSL=0 Compression=0	1m 16s
mysql:latest - SSL=1 Compression=0	1m 24s
mysql:latest - SSL=0 Compression=1	1m 11s
mysql:latest - SSL=1 Compression=1	1m 16s
datagrip/mysql:5.1 - SSL=0 Compression=0	14s
	1d 0h 25m 34s

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