title: Apollo query returns error "Expected String, found String"
body: <p>I'm trying to get single product from DB, but all the time I'm getting the error: 
  "Expected String, found String" from GraphQL.</p>
  
  <p>Here is the code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>class ProductDetail extends React.Component {
    constructor(props) {
      super(props);
    }
  
    render() {
      console.log(JSON.stringify(this.props.id));
      return &lt;Query query={GET_PRODUCT} variables={{_id: JSON.stringify(this.props.id)}}&gt;
      {({ loading, error, data }) =&gt; {
        if (loading) return &lt;ReactLoading type="spin" className="spinner" color={"black"}&gt;&lt;/ReactLoading&gt;
        if (error) return &lt;Alert color="danger"&gt;{error.message}&lt;/Alert&gt;
        return (
          &lt;div&gt;
            {data.products.map(p =&gt; (
              console.log(data)
            ))}
          &lt;/div&gt;
        )
      }}
    &lt;/Query&gt;
    }
  }
  
  
  const GET_PRODUCT = gql`
      {
    product(_id: String) {
      brand
    }
  }
  `
  
  export default ProductDetail;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I noticed $ sign in query defining, tried with that but it returns: 
  "Expected String, found $".</p>