Fix(finalhandler): prevent setting status code after headers sent

[PlumBlossomMaid]

Problem

When a client disconnects prematurely (e.g., rapid page refresh), finalhandler may attempt to set a status code after response headers have already been sent. This triggers an assertion in pegasus:

can't set status code, it was already sent

Fix #5

Root Cause

finalhandler.lua's send() function does not check whether res.pg_res._headersSended is true before calling res:status().

There is already a check for the 404 case at line 80:

if not err and res.pg_res._headersSended then 
    print("cannot 404 after headers sent") 
    return 
end

However, this does not cover cases where err is present (e.g., 500 errors).

Solution

Added a unified check at the beginning of send():

local send = function(req, res, status, headers, msg)
    if res.pg_res._headersSended then
        return
    end
    -- ... existing logic
end

This extends the protection to all error cases and prevents the pegasus assertion entirely.

Testing

• Normal requests: unaffected ✅
• Rapid page refresh: error no longer appears ✅
• Server stability: unchanged ✅

Environment

• OS: Windows 10
• Lua: 5.1.5
• lua-express: 1.0.0
• pegasus: 1.1.0