Update PdfFormField#getFieldName() generation.

If we create PdfString with unicode value we must put `UnicodeBig` encoding.

DEVSIX-1357

Author: pavel-alay

forms/src/main/java/com/itextpdf/forms/PdfAcroForm.java

@@ -845,10 +845,10 @@ private Map<String, PdfFormField> iterateFields(PdfArray array, Map<String, PdfF
                         parentField = PdfFormField.makeFormField(parentField.getParent(), document);
                     }
                 }
-                name = fieldName.getValue() + "." + index;
+                name = fieldName.toUnicodeString() + "." + index;
                 index++;
             } else {
-                name = fieldName.getValue();
+                name = fieldName.toUnicodeString();
             }
             fields.put(name, formField);
             if (formField.getKids() != null) {

forms/src/main/java/com/itextpdf/forms/fields/PdfFormField.java

@@ -1219,7 +1219,7 @@ public PdfString getFieldName() {
         }
         PdfString name = getPdfObject().getAsString(PdfName.T);
         if (name != null) {
-            name = new PdfString(parentName + name.toUnicodeString());
+            name = new PdfString(parentName + name.toUnicodeString(), PdfEncodings.UNICODE_BIG);
         }
         return name;
     }

forms/src/test/java/com/itextpdf/forms/PdfFormFieldTest.java

@@ -137,7 +137,7 @@ public void unicodeFormFieldTest() throws IOException {
         PdfAcroForm form = PdfAcroForm.getAcroForm(pdfDoc, true);
         Map<String, PdfFormField> formFields = form.getFormFields();
         String fieldName = "\u5E10\u53F71"; // 帐号1: account number 1
-        Assert.assertEquals(formFields.keySet().toArray(new String[1])[0], fieldName);
+        Assert.assertEquals(fieldName, formFields.keySet().toArray(new String[1])[0]);
     }
 
     @Test