Issue description

use whole request body to parse graphql will get parse error.
graphql request body is json , example :{"query":"query{getUser{name age}}","variables":null}, not query{getUser{name age}}

Environment

• apisix version (cmd: apisix version): apache/apisix:2.12.0-alpine
• OS (cmd: uname -a): docker
• OpenResty / Nginx version (cmd: nginx -V or openresty -V): null
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API): bitnami/etcd:3.4.15
• apisix-dashboard version, if have: apache/apisix-dashboard:2.10.1-alpine
• the plugin runner version, if the issue is about a plugin runner (cmd: depended on the kind of runner):
• luarocks version, if the issue is about installation (cmd: luarocks --version):

Steps to reproduce

1. define graphql

query {
    getUser:User
}

type User{
    name:String
    age:String
}

2. add route

curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -i -d '
{
    "methods": ["POST"],
    "uri": "/graphql",
    "vars": [
        ["graphql_operation", "==", "query"],
        ["graphql_name", "==", "getUser"]
    ],
    "upstream": {
        "type": "roundrobin",
        "nodes": {
            "127.0.0.1:1980": 1
        }
    }
}'

3. perform graphql request by curl

curl 'http://127.0.0.1:9080/graphql' \
  -H 'Content-Type: text/plain;charset=UTF-8' \
  -H 'Accept: */*' \
  --data-raw '{"query":"query getUser{getUser{name age}}","variables":null}' \
  --compressed

Actual result

HTTP/1.1 404 Not Found
Date: Tue, 08 Feb 2022 07:39:16 GMT
Content-Type: text/plain; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive

{
"error_msg": "404 Route Not Found"
}

Error log

2022/02/08 07:39:16 [error] 45#45: *1085159 [lua] ctx.lua:80: get_parsed_graphql(): failed to parse graphql: Syntax error near line 1 body:

Expected result

success