fix to supress compilation warning on osx mavericks 
fix to supress compilation warning on osx mavericks 

jack-ju/slowhttptest,yeahdongcn/slowhttptest,mfazliazran/slowhttptest,0x37N0w4N/slowhttptest,Chigirin/slowhttptest,mfazliazran/slowhttptest,jengyic/slowhttptest,Chigirin/slowhttptest,tectronics/slowhttptest,tectronics/slowhttptest,mentesc/slowhttptest,yeahdongcn/slowhttptest,mentesc/slowhttptest,jack-ju/slowhttptest,0x37N0w4N/slowhttptest,jengyic/slowhttptest,a12689851/slowhttptest,a12689851/slowhttptest