Fix SetProperty and add unit test.

gtr/gtr.go

@@ -78,7 +78,7 @@ func (p *Package) SetProperty(key, value string) {
 	// then add the specieid key-value property.
 	i := 0
 	for _, prop := range p.Properties {
-		if key == prop.Name {
+		if key != prop.Name {
 			p.Properties[i] = prop
 			i++
 		}

gtr/gtr_test.go

@@ -1,6 +1,10 @@
 package gtr
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
 
 func TestTrimPrefixSpaces(t *testing.T) {
 	tests := []struct {
@@ -24,3 +28,15 @@ func TestTrimPrefixSpaces(t *testing.T) {
 		}
 	}
 }
+
+func TestSetProperty(t *testing.T) {
+	pkg := Package{}
+	pkg.SetProperty("a", "b")
+	pkg.SetProperty("c", "d")
+	pkg.SetProperty("a", "e")
+
+	want := []Property{{Name: "c", Value: "d"}, {Name: "a", Value: "e"}}
+	if diff := cmp.Diff(want, pkg.Properties); diff != "" {
+		t.Errorf("SetProperty got unexpected diff: %s", diff)
+	}
+}