math/big: fix %b format so it matches strconf %b format for non-zero values

math/big: fix %b format so it matches strconf %b format for non-zero values

(For zero values the strconv %b format prints the bias-adjusted exponent;
there's no bias in Float.)

Change-Id: I6f4dda9c3a50d02eac375cfe2c927c1540aae865
Reviewed-on: https://go-review.googlesource.com/3841
Reviewed-by: Alan Donovan <adonovan@google.com>