encoding/asn1: handle UTCTime before the year 2000
encoding/asn1: handle UTCTime before the year 2000

UTCTime only has a two digit date field and year values from 50 should
be 1950, not 2050.

R=golang-dev, r, rsc
CC=golang-dev
http://codereview.appspot.com/5729063

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