title: Flutter graphql query, variables will not update, not sending the latest query
body: <p>I have the following variables set. registerUser is the graphQL api call. I want $name, $password,$premium and $email to be dynamic variables which is determined when a button is pressed (Sign up). This code is all in a stateful widget.</p>
  <pre><code>    String name;
      String password = &quot;&quot;;
      bool premium = false;
      String email = &quot;dasa&quot;;
      String registerUser = '''
      mutation {
        addUserManual(name:&quot;$name&quot;,password:&quot;$password&quot;,premium:$premium,email:&quot;$email&quot;){
        userid
        }
      }
  '''
  </code></pre>
  <p>Then I define a mutation per graphql_flutter plugin like this. In runmutation({.. the mutation sent to the server is that of the initial defined variables above, it does not send the most recent query. Does anyone know how update the vvalue from the fields and then insert this into the query?.</p>
  <pre><code>child: Mutation(options: MutationOptions(
                    documentNode: gql(registerUser),
                    update: (Cache cache, QueryResult result) {
                      return cache;
                    },
                    // or do something with the result.data on completion
                    onCompleted: (dynamic resultData) {
                      print(resultData);
                    },
                  ), builder: (RunMutation runMutation, QueryResult result){
                    return RaisedButton(
                      elevation: 5,
                      onPressed: () {
                        //FIX LATER
                        print(result.data);
                        runMutation({'name':&quot;hi&quot;,'password':&quot;test&quot;,'premium':false,'email':&quot;blabla&quot;});
                      },
                      child: Text('SIGN UP'),
                      color: Colors.white,
                      shape: RoundedRectangleBorder(borderRadius: new BorderRadius.circular(30.0)),
                    );
                  })
  </code></pre>