8218554: HttpServer: allow custom handlers to request that the connection be closed after the exchange. Summary: custom handler code can supply `Connection: close` to response headers in order to force connection close after the exchange terminates. Reviewed-by: chegar

/build/
/dist/
/.idea/
nbproject/private/
/webrev
/.src-rev
/.jib/
.DS_Store
.metadata/
.recommenders/
test/nashorn/script/external
test/nashorn/lib
NashornProfile.txt
**/JTreport/**
**/JTwork/**