title: DIO response decode issue
body: <p>I am using <code>Dio</code> for making the <code>HTTP</code> request</p>
  
  <pre><code>  var dio = Dio();
      var response =
          await dio.get(URL);
      final responseBody = json.decode(response.data);
  
      final statusCode = response.statusCode;
  
      if (statusCode != 200 || responseBody == null) {
        print("status code:$statusCode");
        throw new ServerExceptionHandler(
            "An error ocurred : [Status Code : $statusCode]", statusCode);
      }
  </code></pre>
  
  <blockquote>
    <p>A response I am paring</p>
  </blockquote>
  
  <pre><code>{
  "x_id": "home"
  }
  </code></pre>
  
  <p>but <code>json.decode(response.data)</code> is throwing the exception.</p>
  
  <pre><code>type '_InternalLinkedHashMap&lt;String, dynamic&gt;' is not a subtype of type 'String'
  </code></pre>
  
  <p>The Strange thing is when I replace <code>Dio</code> with <code>http.get(url)</code> then <code>json.decode(response.body)</code> works perfectly with same response</p>