only add application/xml content type when needed

package.go

@@ -46,11 +46,7 @@ func (p *pkg) add(part *Part) error {
 	if err := part.validate(); err != nil {
 		return err
 	}
-	name := part.Name
-	if !strings.EqualFold(name, contentTypesName) {
-		name = NormalizePartName(part.Name)
-	}
-	name = strings.ToUpper(name)
+	name := strings.ToUpper(NormalizePartName(part.Name))
 	if p.partExists(name) {
 		return newError(112, part.Name)
 	}

writer.go

@@ -41,7 +41,6 @@ func NewWriter(w io.Writer) *Writer {
 		parts: make(map[string]struct{}, 0),
 		contentTypes: contentTypes{
 			defaults: map[string]string{
-				"xml":  "application/xml",
 				"rels": relationshipContentType,
 			},
 			overrides: map[string]string{},
@@ -165,7 +164,12 @@ func (w *Writer) createCoreProperties() error {
 
 func (w *Writer) createContentTypes() error {
 	// ISO/IEC 29500-2 M3.10
-	cw, err := w.addToPackage(&Part{Name: contentTypesName, ContentType: "application/xml"}, CompressionNormal)
+	fh := &zip.FileHeader{
+		Name:     zipName(contentTypesName),
+		Modified: time.Now(),
+	}
+	w.setCompressor(fh, CompressionNormal)
+	cw, err := w.w.CreateHeader(fh)
 	if err != nil {
 		return err
 	}

writer_test.go

@@ -27,8 +27,6 @@ func TestWriter_Close(t *testing.T) {
 	p := newPackage()
 	p.contentTypes.add("/a.xml", "a/b")
 	p.contentTypes.add("/b.xml", "c/d")
-	pC := newPackage()
-	pC.parts["/[CONTENT_TYPES].XML"] = struct{}{}
 	pCore := newPackage()
 	pCore.parts["/PROPS/CORE.XML"] = struct{}{}
 	pRel := newPackage()
@@ -39,7 +37,6 @@ func TestWriter_Close(t *testing.T) {
 		wantErr bool
 	}{
 		{"base", NewWriter(&bytes.Buffer{}), false},
-		{"invalidContentType", &Writer{p: pC, w: zip.NewWriter(&bytes.Buffer{})}, true},
 		{"withCt", &Writer{p: p, w: zip.NewWriter(&bytes.Buffer{})}, false},
 		{"invalidPartRel", &Writer{p: newPackage(), w: zip.NewWriter(&bytes.Buffer{}), last: &Part{Name: "/b.xml", Relationships: []*Relationship{{}}}}, true},
 		{"invalidOwnRel", &Writer{p: newPackage(), w: zip.NewWriter(&bytes.Buffer{}), Relationships: []*Relationship{{}}}, true},