title: Unable to pass variable data to a GraphQL mutation
body: <p>My Strapi installation has a GraphQL endpoint at <a href="https://dev.schandillia.com/graphql" rel="nofollow noreferrer">https://dev.schandillia.com/graphql</a>.</p>
  
  <p>Here, I'm trying to execute the following mutation:</p>
  
  <pre><code>mutation updatePost ($id: ID!, $title: String!){
    updatePost(input: {where: {id: $id}, data: {title: $title}}){
      post{
        id
        title
      }
    }
  }
  </code></pre>
  
  <p>As can be seen, this mutation accepts 2 variables. These I am providing thus:</p>
  
  <pre><code>{
    "id": "5ddf9ff327ee7b0915f0ac91"
    "title": "Some random title"
  }
  </code></pre>
  
  <p>For the endpoint to work, the following auth header will be required:</p>
  
  <pre><code>{
    "Authorization": "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjVkOWVlNDBhNDYyMGNiMzBiZjdhZGVlMyIsImlzQWRtaW4iOnRydWUsImlhdCI6MTU4NTIxNTU2OCwiZXhwIjoxNTg3ODA3NTY4fQ.0xrjVB9cKxiqN6FbvQ-sRV7nmAHNrV_mSD8D7e1g3qw"
  }
  </code></pre>
  
  <p>It's a very simple mutation using which I am attempting to update a post's title field to <code>Some random title</code>. The post is being looked up by its id field which should be <code>5ddf9ff327ee7b0915f0ac91</code>.</p>
  
  <p>However, this attempt fails with the following output:</p>
  
  <pre><code>{
    "error": {
      "errors": [
        {
          "message": "Variable \"$id\" of required type \"ID!\" was not provided.",
          "locations": [
            {
              "line": 1,
              "column": 21
            }
          ],
          "extensions": {
            "code": "INTERNAL_SERVER_ERROR"
          }
        },
        {
          "message": "Variable \"$title\" of required type \"String!\" was not provided.",
          "locations": [
            {
              "line": 1,
              "column": 31
            }
          ],
          "extensions": {
            "code": "INTERNAL_SERVER_ERROR"
          }
        }
      ]
    }
  }
  </code></pre>
  
  <p>What am I missing here? It says I've not provided it the variables, but I did!</p>
  
  <p><a href="https://i.stack.imgur.com/Kva1A.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Kva1A.jpg" alt="The GraphQL endpoint in action"></a></p>