net/http: fix http_proxy parsing
net/http: fix http_proxy parsing

Fixes issue 2919.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5645089

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