Fixed bug that caused Exp(x, y, m) ( i.e. x**y (mod m) ) to return x
instead of x (mod m) when y == 1. See issue page on github for more
details.

Added test case

Fixes #9826

Change-Id: Ibabb58275a20c4231c9474199b7f1c10e54241ce
Reviewed-on: https://go-review.googlesource.com/8409Reviewed-by: Robert Griesemer <gri@golang.org>