fix: unable to use variable as graphql input

[yanghua-ola]

This PR add the ability to use variable as GraphQL query variables, e.g.

{{
  exports.variables = {
    "limit": 10
  };
}}

POST  http://localhost:7002/graphql

query launchesQuery($limit: Int!){
  launchesPast(limit: $limit) {
    mission_name
  }
}

{{ variables }}

OR

{{
  exports.variables = {
    "foo": 10,
    "bar": 20
  };
}}

POST  http://localhost:7002/graphql

query launchesQuery($limit: ComplexInput!){
  launchesPast(limit: $limit) {
    mission_name
  }
}

{
  "limit": {{ variables }}
}

Above requests raise exception in current version, because JSON.parse was hooked on execute event, which happens before variable replacement.

[yanghua-ola]

Wait, just noticed it broke for loop

[yanghua-ola]

Wait, just noticed it broke for loop

fixed

[AnWeber]

Yes makes sense to parse it only after the variable substitution. Thanks for the improvement.