title: UnityWebRequest POST to PHP not work
body: <p>I use documentation with WWWForm: <a href="https://docs.unity3d.com/Manual/UnityWebRequest-SendingForm.html" rel="nofollow noreferrer">https://docs.unity3d.com/Manual/UnityWebRequest-SendingForm.html</a></p>
  
  <p>C#:</p>
  
  <pre><code>void Start() {
      StartCoroutine(Upload());
  }
  
  IEnumerator Upload(){
      yield return Upload1();
      yield return Upload2();
  }
  
  IEnumerator Upload1() {
      List&lt;IMultipartFormSection&gt; formData = new List&lt;IMultipartFormSection&gt;();
      formData.Add( new MultipartFormDataSection("field1=foo&amp;field2=bar") );
      //formData.Add( new MultipartFormFileSection("my file data", "myfile.txt") );
  
      UnityWebRequest www = UnityWebRequest.Post(url, formData);
      yield return www.SendWebRequest();
  
      if(www.isNetworkError || www.isHttpError) {
          Debug.Log(www.error);
      }
      else {
          Debug.Log("Form upload complete!");
          Debug.Log( "MULTIPART: " + www.downloadHandler.text );
      }
  }
  
  IEnumerator Upload2() {
      WWWForm form = new WWWForm();
      form.AddField("myField", "myData");
  
      UnityWebRequest www = UnityWebRequest.Post(url, form);
      yield return www.SendWebRequest();
  
      if(www.isNetworkError || www.isHttpError) {
          Debug.Log(www.error);
      }
      else {
          Debug.Log("Form upload complete!");
          Debug.Log( "WWWForm: " + www.downloadHandler.text );
      }
  }
  </code></pre>
  
  <p>PHP:</p>
  
  <pre><code>&lt;?php 
  
      echo "POST: ";
      print_r( $_POST );
      var_dump( $_POST );
  
      echo "GET: ";
      print_r( $_GET );
      var_dump( $_GET );
  
  ?&gt;
  </code></pre>
  
  <p>Response (MULTIPART and WWWForm):</p>
  
  <pre><code>POST: Array
  (
  )
  array(0) {
  }
  GET: Array
  (
  )
  array(0) {
  }
  </code></pre>
  
  <h1>$_POST is empty... (that's my problem)</h1>
  
  <p>Use Unity 2017.3.</p>
  
  <p>I used UnityWebRequest.Send (Unity 5.6) before and it worked for me until it became obsolete.</p>
  
  <p>Thanks.</p>