math/big: Exp function returns puzzling results when exponent = 1

[royalfork]

I'm using Go 1.4 on 64 bit Debian.

I run the following:
import "math/big"
a := new(big.Int).Exp(big.NewInt(5), big.NewInt(1), big.NewInt(3))

I expect 'a' to be 2, because 5**1 % 3 = 5 % 3 = 2. Instead, Go gives a = 5.

Code can be found here: http://play.golang.org/p/TCWWiXXZhf

[ALTree]

In nat.go expNN (here), when y is 1, v is set to 1 (here) and shift on the next line to 64. Then on the line

w := _W - int(shift)

w is set to 0 (64 - 64), so the code inside the loop on line 921 is never ran. Since z (the result) was set to x (on line 895) and never changed the function returns x (5 in the example) instead of x mod m.

Patch: https://go-review.googlesource.com/#/c/8409