Why office365 xlsx sheet utf8 encoding but openxmlsdk is utf8-withbom

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- [X] Issue with the OpenXml library
- [ ] Question on library usage

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**Description**

Please provide a simple description of the issue encountered.

**Information**

- .NET Target: .NET Core
- DocumentFormat.OpenXml Version: 2.14.0

**Repro**

```csharp
public class Program
{
public static void Main(string[] args)
{
var directoryInfo = new DirectoryInfo(Directory.GetCurrentDirectory());
var fileName = $@"PracticePart1-{DateTime.Now:yyyyMMddHHmmss}.xlsx";
var filepath = Path.Combine(directoryInfo.ToString(), fileName);
Console.WriteLine($"FilePath: {filepath}");
var spreadsheetDocument = SpreadsheetDocument.Create(filepath, SpreadsheetDocumentType.Workbook);
var workbookPart = spreadsheetDocument.AddWorkbookPart();
workbookPart.Workbook = new Workbook();
var worksheetPart = workbookPart.AddNewPart();
worksheetPart.Worksheet = new Worksheet(new SheetData());
var sheets = spreadsheetDocument.WorkbookPart.Workbook.AppendChild(new Sheets());
var sheet = new Sheet()
{
Id = spreadsheetDocument.WorkbookPart.GetIdOfPart(worksheetPart),
SheetId = 1,
Name = "myFirstSheet"
};
sheets.Append(sheet);
workbookPart.Workbook.Save();
spreadsheetDocument.Close();
}
}

```

**Observed**

Office 365 encoding are all utf-8 without bom, but openxmlsdk some're utf-8 with bom and some not

**Expected**

Should we follow office365 encoding standard? (below image is office 365 xlsx)

[office365_sample.xlsx](https://github.com/OfficeDev/Open-XML-SDK/files/7490064/office365_sample.xlsx)