title: iText doesn't set checkbox field
body: <p>I'm using iText library to generate pdf file based on template. But when I try to set checkbox field it won't filled. 
  Class is here:</p>
  
  <pre><code>public class MainClass {
      public static void main(String[] args) {
          try {
              PdfReader reader = new PdfReader("pdf/fw9_template.pdf");
              PdfStamper stamper = new PdfStamper(reader, new FileOutputStream("test.pdf"));
              AcroFields form = stamper.getAcroFields();
  
              String states[] = form.getAppearanceStates("topmostSubform[0].Page1[0].FederalClassification[0].c1_1");
              System.out.println(states);
  
  
              for (Iterator i = form.getFields().keySet().iterator(); i.hasNext(); ) {
                  String key = (String) i.next();
                  System.out.print(key + " : ");
                  switch(form.getFieldType(key)) {
                      case AcroFields.FIELD_TYPE_CHECKBOX:
                          System.out.println("Checkbox");
                          break;
                      case AcroFields.FIELD_TYPE_COMBO:
                          System.out.println("Combobox");
                          break;
                      case AcroFields.FIELD_TYPE_LIST:
                          System.out.println("List");
                          break;
                      case AcroFields.FIELD_TYPE_NONE:
                          System.out.println("None");
                          break;
                      case AcroFields.FIELD_TYPE_PUSHBUTTON:
                          System.out.println("Pushbutton");
                          break;
                      case AcroFields.FIELD_TYPE_RADIOBUTTON:
                          System.out.println("Radiobutton");
                          break;
                      case AcroFields.FIELD_TYPE_SIGNATURE:
                          System.out.println("Signature");
                          break;
                      case AcroFields.FIELD_TYPE_TEXT:
                          System.out.println("Text");
                          break;
                      default:
                          System.out.println("?");
                  }
              }
              form.setField("topmostSubform[0].Page1[0].FederalClassification[0].c1_1[0]", "true");
  
              stamper.close();
          } catch (Exception e) {
              System.out.println(e);
          }
      }
  }
  </code></pre>
  
  <p>Program output:</p>
  
  <pre><code>null
  topmostSubform[0].Page1[0].EmployerID[0].f1_14[0] : Text
  topmostSubform[0].Page1[0].FederalClassification[0].f1_4[0] : Text
  topmostSubform[0].Page1[0].f1_9[0] : Text
  topmostSubform[0].Page1[0].f1_10[0] : Text
  topmostSubform[0].Page1[0].FederalClassification[0].c1_1[1] : Checkbox
  topmostSubform[0].Page1[0].f1_2[0] : Text
  topmostSubform[0].Page1[0].EmployerID[0].f1_15[0] : Text
  topmostSubform[0].Page1[0].SSN[0].f1_12[0] : Text
  topmostSubform[0].Page1[0].SSN[0].f1_13[0] : Text
  topmostSubform[0].Page1[0].Address[0].f1_8[0] : Text
  topmostSubform[0].Page1[0].Address[0].f1_7[0] : Text
  topmostSubform[0].Page1[0].FederalClassification[0].c1_1[0] : Checkbox
  topmostSubform[0].Page1[0].FederalClassification[0].c1_1[2] : Checkbox
  topmostSubform[0].Page1[0].FederalClassification[0].c1_7[0] : Checkbox
  topmostSubform[0].Page1[0].FederalClassification[0].c1_1[4] : Checkbox
  topmostSubform[0].Page1[0].FederalClassification[0].c1_1[3] : Checkbox
  topmostSubform[0].Page1[0].f1_1[0] : Text
  topmostSubform[0].Page1[0].FederalClassification[0].c1_1[5] : Checkbox
  topmostSubform[0].Page1[0].SSN[0].f1_11[0] : Text
  topmostSubform[0].Page1[0].FederalClassification[0].f1_3[0] : Text
  topmostSubform[0].Page1[0].Exemptions[0].f1_6[0] : Text
  topmostSubform[0].Page1[0].Exemptions[0].f1_5[0] : Text
  </code></pre>
  
  <p>I don't understand why the method getAppearanceStates() returns 'null', when we can see checkboxes in the output. Also checkbox doesn't filled in a result pdf file.</p>
  
  <p>Can anybody help me to resolve this issue?</p>
  
  <p>Thanks in advance,
  Sergey</p>