error with PDFTextExtractor

[Wugengxian]

Describe the bug
/toUnicode is error, then PDFTextExtractor will make mistake.

To Reproduce
this is code:

void testToUnicode() throws Exception {
        Document document = new Document();
        Document.compress = false;
        FileOutputStream outputStream = new FileOutputStream("output.pdf");
        PdfWriter.getInstance(document, outputStream);
        document.open();

        document.add(new Chunk("ετε", new Font(Font.SYMBOL)));
        document.close();
        PdfTextExtractor pdfTextExtractor = new PdfTextExtractor(new PdfReader("output.pdf"));
        Assertions.assertEquals("ετε", pdfTextExtractor.getTextFromPage(1));
    }

Expected behavior
when we copy "ετε" in html or use PdfTextExtractor, it show "ͧͶͧ". which is error
Expected behavior
when we copy "ετε" in html or use PdfTextExtractor, it should show "ετε".

Screenshots

System (please complete the following information):

• OS: Windows 10
• Used Font:

Additional context
I have fixed it, the error happen in /ToUnicode.
error /ToUnicode:

[Wugengxian]

it also has other problem when we use font.Symbol in Chunk. I will provide a pull request in the future.