BUG/MEDIUM: fix serialization of FCGI option mpxs-conns #1016

Job	Run time
generate-models	1m 42s
generate-specification	1m 0s
HAProxy check commit message	0s
lint-spec	11s
golangci-lint	1m 15s
go mod tidy	19s
Build	34s
license-check	29s
Tests	54s
Test equal	1m 23s
e2e on HAProxy (2.2)	55s
e2e on HAProxy (2.3)	53s
e2e on HAProxy (2.1)	51s
e2e on HAProxy (2.6)	50s
e2e on HAProxy (2.5)	52s
e2e on HAProxy (2.8)	51s
e2e on HAProxy (2.7)	1m 10s
e2e on HAProxy (2.4)	1m 7s
	15m 16s

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