html/template: include itself while cloning

template.Clone() initialized template set incorrectly:
it didn't include itself.

* include itself in template set while cloning
* add a test

Fixes #12996

Change-Id: I932530e4f7f1bbebf833e12b000a5ce052bc9223
Reviewed-on: https://go-review.googlesource.com/16104Reviewed-by: Andrew Gerrand <adg@golang.org>

src/html/template/template.go

@@ -230,6 +230,7 @@ func (t *Template) Clone() (*Template, error) {
 			set: make(map[string]*Template),
 		},
 	}
+	ret.set[ret.Name()] = ret
 	for _, x := range textClone.Templates() {
 		name := x.Name()
 		src := t.set[name]

src/html/template/template_test.go

+package template
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestTemplateClone(t *testing.T) {
+	// https://golang.org/issue/12996
+	orig := New("name")
+	clone, err := orig.Clone()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(clone.Templates()) != len(orig.Templates()) {
+		t.Fatalf("Invalid lenth of t.Clone().Templates()")
+	}
+
+	const want = "stuff"
+	parsed := Must(clone.Parse(want))
+	var buf bytes.Buffer
+	err = parsed.Execute(&buf, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want {
+		t.Fatalf("got %q; want %q", got, want)
+	}
+}