check for x-response-json header before attempting to use it

[notnmeyer]

here we read the header directly without verifying if it exists yet first. this causes the request to blow up and 502 if its behind an LB,

➜ curl -i https://mockpi.fly.dev
HTTP/2 502
server: Fly/55f8eaa0 (2023-10-09)
via: 2 fly.io
fly-request-id: 01HDF07QYT6S44KJKSAJ6DPEE2-sea
date: Mon, 23 Oct 2023 19:29:53 GMT

and the error itself,

➜ go run main.go
Listening on :8080...
2023/10/23 13:57:27 http: panic serving 127.0.0.1:56990: runtime error: index out of range [0] with length 0
goroutine 4 [running]:
net/http.(*conn).serve.func1()
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/net/http/server.go:1868 +0xb0
panic({0x1025eb040?, 0x1400001a300?})
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/runtime/panic.go:920 +0x26c
main.handler({0x102607bc0, 0x14000152000}, 0x14000130100)
        /Users/nate/code/mockpi/main.go:37 +0x240
net/http.HandlerFunc.ServeHTTP(0x14000149b08?, {0x102607bc0?, 0x14000152000?}, 0x1024fdb48?)
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/net/http/server.go:2136 +0x38
net/http.(*ServeMux).ServeHTTP(0x1027c63a0?, {0x102607bc0, 0x14000152000}, 0x14000130100)
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/net/http/server.go:2514 +0x144
net/http.serverHandler.ServeHTTP({0x1400007ec30?}, {0x102607bc0?, 0x14000152000?}, 0x6?)
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/net/http/server.go:2938 +0xbc
net/http.(*conn).serve(0x14000140000, {0x1026083f8, 0x1400007eb40})
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/net/http/server.go:2009 +0x518
created by net/http.(*Server).Serve in goroutine 1
        /nix/store/s1xk6hrfn0kw8ycvqbx4vv4gr00k6qpl-go-1.21.1/share/go/src/net/http/server.go:3086 +0x4cc

we should check for the presence of the header first and return a 400 Bad Request when it's missing instead

[notnmeyer]

fixed in #15