QHttpServer statusString map is missing a few of status codes

qhttpserverresponder.cpp:42 statusString map is missing a few of the status code 'translations' e.g. UpgradeRequired, ImATeapot (and others), which results in segfault upon serving the request (qhttpserverresponder.cpp:374).

• Add missing statusCode-string entries to the statusString map
• add check  does stastusString map have the key before calling statusString.at() in writeStatusLine()