Fix page dict string value corruption in createIncPageUpdate

[fredgig]

Bug

createIncPageUpdate writes page dictionary string values (like /LastModified, /CreationDate) using Go double quotes instead of PDF parenthesized strings.

This breaks multi-signature PDFs. After the first signature rewrites the page dict, subsequent calls to pdf.NewReader panic:

unexpected keyword "\"D:20220711015152-06'00'\"" parsing object

Root Cause

The default case uses page.Key(key).String() to serialize values. Value.String() calls objfmt(), which runs strconv.Quote() on string values, producing "text" instead of (text).

default:
    page_buffer.WriteString(fmt.Sprintf("  /%s %s\n", key, page.Key(key).String()))

A page entry like:

/LastModified (D:20220711015152-06'00')

Gets rewritten as:

/LastModified "D:20220711015152-06'00'"

Double quotes aren't valid PDF string syntax. The lexer treats it as an unexpected keyword and panics.

Repro

1. Take a PDF with a string value in a page dictionary (e.g. /LastModified)
2. Sign with 2+ signature fields on pages containing that string
3. First signature corrupts the string, second signature panics on parse

Fix

Check for pdf.String kind and use pdfString(val.RawString()) instead of val.String(). pdfString() in sign/helpers.go escapes \, (, ), \r and wraps in (...). RawString() gives the unescaped bytes, pdfString() re-escapes for valid PDF output.

Fixes #132

[vanbroup]

Please note that #126 is going te replace the current main branch when ready.