httptest: default the Recorder status code to 200 on a Write
httptest: default the Recorder status code to 200 on a Write

This matches the real ResponseWriter behavior.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4291048

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