Update PdfFormField#getFieldName() generation.

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

DEVSIX-1357

Autoported commit.
Original commit hash: [8b8819c7f]

Author: pavel-alay

itext.tests/itext.forms.tests/itext/forms/PdfFormFieldTest.cs

@@ -123,7 +123,7 @@ public virtual void UnicodeFormFieldTest() {
             IDictionary<String, PdfFormField> formFields = form.GetFormFields();
             String fieldName = "\u5E10\u53F71";
             // 帐号1: account number 1
-            NUnit.Framework.Assert.AreEqual(formFields.Keys.ToArray(new String[1])[0], fieldName);
+            NUnit.Framework.Assert.AreEqual(fieldName, formFields.Keys.ToArray(new String[1])[0]);
         }
 
         /// <exception cref="System.IO.IOException"/>

itext/itext.forms/itext/forms/PdfAcroForm.cs

@@ -952,11 +952,11 @@ private IDictionary<String, PdfFormField> IterateFields(PdfArray array, IDiction
                             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) {

itext/itext.forms/itext/forms/fields/PdfFormField.cs

@@ -1583,7 +1583,7 @@ public virtual 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;
         }

port-hash

@@ -1 +1 @@
-30135114afcdbee5e6061a4ffb5c8b948091d1dc
+8b8819c7f336e9c986cc7e36de625e97905b509b