time: in Format give buffer an initial capacity

time: in Format give buffer an initial capacity

I have a small web server that simply sets several cookies
along with an expires header, and then returns.  In the
cpuprofile for a 200k request benchmark, time.Time.Format()
was showing up as 8.3% of cpu usage.  Giving the buffer an
inital capacity to avoid reallocs on append drops it down to
7.6%.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5992058