http: avoid panic caused by nil URL
http: avoid panic caused by nil URL

The current code will panic if an invalid
request (one with a nil URL) is passed to
the doFollowingRedirects function.

Also, remove a redundant nil Header check.

R=bradfitz
CC=golang-dev
http://codereview.appspot.com/5270046

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang