This closes #1269, made the NewStreamWriter function case insensitive to worksheet name

Co-authored-by: xiangyz <xiangyz@dustess.com>

Author: eaglexiang

sheet.go

@@ -376,7 +376,7 @@ func (f *File) GetSheetName(index int) (name string) {
 // integer type value -1.
 func (f *File) getSheetID(name string) int {
 	for sheetID, sheet := range f.GetSheetMap() {
-		if sheet == trimSheetName(name) {
+		if strings.EqualFold(sheet, trimSheetName(name)) {
 			return sheetID
 		}
 	}

sheet_test.go

@@ -465,6 +465,13 @@ func TestDeleteAndAdjustDefinedNames(t *testing.T) {
 	deleteAndAdjustDefinedNames(&xlsxWorkbook{}, 0)
 }
 
+func TestGetSheetID(t *testing.T) {
+	file := NewFile()
+	file.NewSheet("Sheet1")
+	id := file.getSheetID("sheet1")
+	assert.NotEqual(t, -1, id)
+}
+
 func BenchmarkNewSheet(b *testing.B) {
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {