graphql: set content-type as json

[jsvisa]

Fix an issue of wrong Content-Type

$ curl -XPOST 127.0.0.1:8545/graphql -d '{"query": "{block(number: 1  hash: \"0xc1d1224983a2b43f2ea1b847337eda55e316e94603386e346abe3f6e08121027\") {number hash }}"}' -v
Note: Unnecessary use of -X or --request, POST is already inferred.
* Connected to 127.0.0.1 (127.0.0.1) port 8545 (#0)
> POST /graphql HTTP/1.1
> Host: 127.0.0.1:8545
> User-Agent: curl/8.1.2
> Accept: */*
> Content-Length: 124
> Content-Type: application/x-www-form-urlencoded
>
< HTTP/1.1 400 Bad Request
< Date: Thu, 26 Oct 2023 00:29:18 GMT
< Content-Length: 110
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
{"errors":[{"message":"only one of number or hash must be specified","path":["block"]}],"data":{"block":null}}

@s1na PTAL