BUG/MEDIUM: fix serialization of FCGI option mpxs-conns #1017

Job	Run time
generate-specification	1m 0s
generate-models	1m 39s
HAProxy check commit message	0s
Build	0s
go mod tidy	0s
lint-spec	0s
golangci-lint	0s
license-check	0s
Test equal	0s
Tests	0s
e2e on HAProxy	-1s
	2m 38s

Provide feedback