create XML coverage file without BOM

[Bertk]

UTF-8. The UTF-8 representation of the BOM is the (hexadecimal) byte sequence EF BB BF . The Unicode Standard permits the BOM in UTF-8, but does not require or recommend its use.

closes #1602

[daveMueller]

Wouldn't it make sense to add a unit test for reports without BOM? So that somebody else who might work on this knows that we do this for a reason. Sth. like:

[Fact]
public void CoberturaTestReportDoesNotContainBom()
{
  var result = new CoverageResult { Parameters = new CoverageParameters(), Identifier = Guid.NewGuid().ToString() };
  var documents = new Documents();
  var classes = new Classes { { "Class", new Methods() } };

  documents.Add(RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? @"C:\doc.cs" : @"/doc.cs", classes);

  result.Modules = new Modules { { "Module", documents } };

  var reporter = new CoberturaReporter();
  string report = reporter.Report(result, new Mock<ISourceRootTranslator>().Object);

  byte[] preamble = Encoding.UTF8.GetBytes(report)[..3];
  Assert.NotEqual(Encoding.UTF8.GetPreamble(), preamble);
}