http: fix text displayed in Redirect
http: fix text displayed in Redirect

In the case where r.Method == "POST", was
calling Printf with an argument but "" format string,
causing a spurious %!EXTRA(...) message.

Also escape string properly in HTML generation.

R=r
CC=golang-dev
http://codereview.appspot.com/3923043

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