[web] fix timing display for keepalive connections
[web] fix timing display for keepalive connections

see https://discourse.mitmproxy.org/t/mitmweb-details-tab-timing/848/3
vhaupert/mitmproxy,mhils/mitmproxy,ujjwal96/mitmproxy,mhils/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,zlorb/mitmproxy,vhaupert/mitmproxy,mitmproxy/mitmproxy,mitmproxy/mitmproxy,ujjwal96/mitmproxy,mhils/mitmproxy,vhaupert/mitmproxy,Kriechi/mitmproxy,zlorb/mitmproxy,Kriechi/mitmproxy,cortesi/mitmproxy,Kriechi/mitmproxy,mitmproxy/mitmproxy,mhils/mitmproxy,cortesi/mitmproxy,mhils/mitmproxy,ujjwal96/mitmproxy,zlorb/mitmproxy,vhaupert/mitmproxy,cortesi/mitmproxy,zlorb/mitmproxy,cortesi/mitmproxy,mitmproxy/mitmproxy,ujjwal96/mitmproxy