gerrit: add SetReview method

Tested it at: https://go-review.googlesource.com/#/c/4460/
(the 3rd comment from gobot, at minute :18)

Change-Id: If42eebf057b15ce83d1d4ed2e14460011a8d02d4
Reviewed-on: https://go-review.googlesource.com/4532
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/gerrit/demo.go b/gerrit/demo.go
index 95b576e..62a9f55 100644
--- a/gerrit/demo.go
+++ b/gerrit/demo.go
@@ -33,4 +33,11 @@
 	}
 	v, _ := json.MarshalIndent(cl, "", "  ")
 	os.Stdout.Write(v)
+
+	log.Printf("SetReview = %v", c.SetReview("I2383397c056a9ffe174ac7c2c6e5bb334406fbf9", "current", gerrit.ReviewInput{
+		Message: "test test",
+		Labels: map[string]int{
+			"TryBot-Result": -1,
+		},
+	}))
 }