http: on invalid request, send 400 response
http: on invalid request, send 400 response

Fixes issue 2160

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/4930049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang