kubernetes: Add zone filtering to fallthrough (#1353)

* Add zone filtering to fallthrough

* Doh. gofmt

* Update documentation

plugin/plugin_test.go

@@ -1 +1,21 @@
 package plugin
+
+import "testing"
+
+func TestFallthrough(t *testing.T) {
+	if Fallthrough(nil, "foo.com.") {
+		t.Errorf("Expected false, got true for nil fallthrough")
+	}
+
+	if !Fallthrough(&[]string{}, "foo.net.") {
+		t.Errorf("Expected true, got false for all zone fallthrough")
+	}
+
+	if Fallthrough(&[]string{"foo.com", "bar.com"}, "foo.net") {
+		t.Errorf("Expected false, got true for non-matching fallthrough zone")
+	}
+
+	if !Fallthrough(&[]string{"foo.com.", "bar.com."}, "bar.com.") {
+		t.Errorf("Expected true, got false for matching fallthrough zone")
+	}
+}