Fix #64878: 304 responses return Content-Type header

According to RFC 7232 304 responses should not send a Content-Type header,
so the CLI server should comply.

Author: cmb69

sapi/cli/php_cli_server.c

@@ -2185,6 +2185,9 @@ static int php_cli_server_dispatch(php_cli_server *server, php_cli_server_client
 	if (!is_static_file) {
 		if (SUCCESS == php_cli_server_dispatch_script(server, client TSRMLS_CC)
 				|| SUCCESS != php_cli_server_send_error_page(server, client, 500 TSRMLS_CC)) {
+			if (SG(sapi_headers).http_response_code == 304) {
+				SG(sapi_headers).send_default_content_type = 0;
+			}
 			php_cli_server_request_shutdown(server, client TSRMLS_CC);
 			return SUCCESS;
 		}

sapi/cli/tests/bug64878.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Bug #64878 (304 responses return Content-Type header)
+--INI--
+allow_url_fopen=1
+--SKIPIF--
+<?php
+include "skipif.inc";
+?>
+--FILE--
+<?php
+include "php_cli_server.inc";
+php_cli_server_start('header("HTTP/1.1 304 Not Modified")', null);
+$headers = get_headers('http://' . PHP_CLI_SERVER_ADDRESS);
+echo count(array_filter($headers, function ($value) {
+    return stripos($value, 'Content-Type') === 0;
+}));
+?>
+--EXPECT--
+0