Enhance send to stream special files safely

[godwincod3s]

Add allowSpecialFiles option to support streaming pseudo‑files (e.g. /proc).

Summary
This PR introduces a new opt‑in option, allowSpecialFiles, to the send module. It addresses the issue where attempting to download pseudo‑files (e.g. /proc/meminfo on Linux) results in empty responses because fs.stat reports a size of 0.

Problem

• send relies on fs.stat.size to set Content-Length.
• For virtual filesystems like /proc, stat.size = 0, so the response is empty.
• Developers currently have to bypass res.download() and use fs.createReadStream manually.

Solution

• Added _allowSpecialFiles flag in SendStream constructor (default: false).
• In SendStream.prototype.send, detect when stat.size === 0 and _allowSpecialFiles is enabled.
• Skip setting Content-Length and stream the file directly.
• Maintains existing behavior unless explicitly enabled.

Usage Example

app.get('/download', (req, res) => {
  res.download('/proc/meminfo', 'meminfo.txt', { allowSpecialFiles: true }, (err) => {
    if (err) console.error('Download error:', err)
  })
})

Security Considerations

• By default, allowSpecialFiles is disabled.
• /proc and similar pseudo‑files can expose sensitive system information.
• Developers must consciously enable this option, ensuring safe defaults for all existing applications.

Tests Added

• /proc/meminfo with allowSpecialFiles: true → non‑empty response containing "MemTotal".
• /proc/meminfo without flag → empty response (unchanged behavior).