veb: fix unset Content-Length header, in responses with an empty body (#26431)

Author: D4MI4NX

vlib/veb/context.v

@@ -115,7 +115,7 @@ pub fn (mut ctx Context) send_response_to_client(mimetype string, response strin
 	if custom_mimetype != '' {
 		ctx.res.header.set(.content_type, custom_mimetype)
 	}
-	if ctx.res.body != '' {
+	if !ctx.res.header.contains(.content_length) {
 		ctx.res.header.set(.content_length, ctx.res.body.len.str())
 	}
 	// send veb's closing headers

vlib/veb/tests/veb_test.v

@@ -297,6 +297,17 @@ fn test_host() {
 	assert x.status() == .ok
 }
 
+fn test_empty_response_body_has_content_length() {
+	req := http.Request{
+		url:    'http://${localserver}/empty_response_body'
+		method: .get
+	}
+
+	mut x := req.do()!
+	assert x.status() == .ok
+	assert x.header.get(.content_length)! == '0'
+}
+
 fn test_http_client_shutdown_does_not_work_without_a_cookie() {
 	x := http.get('http://${localserver}/shutdown') or {
 		assert err.msg() == ''

vlib/veb/tests/veb_test_server.v

@@ -136,6 +136,11 @@ pub fn (mut app ServerApp) with_host(mut ctx ServerContext) veb.Result {
 	return ctx.ok('')
 }
 
+@['/empty_response_body']
+pub fn (mut app ServerApp) empty_response_body(mut ctx ServerContext) veb.Result {
+	return ctx.ok('')
+}
+
 pub fn (mut app ServerApp) shutdown(mut ctx ServerContext) veb.Result {
 	session_key := ctx.get_cookie('skey') or { return ctx.not_found() }
 	if session_key != 'superman' {