title: XmlTextWriter default encoding behaves differently to setting encoding to UTF-8
body: <p>I am seeing some behaviour I don't expect with XmlTextWriter. When I specifying the encoding  when I instantiate the writer by either</p>
  
  <pre><code>new XmlTextWriter(fs, Encoding.UTF8) 
  </code></pre>
  
  <p>or</p>
  
  <pre><code>XmlWriter.Create(fs, new XmlWriterSettings(){Encoding = Encoding.UTF8} )
  </code></pre>
  
  <p>the document produced has a leading hex character at the start of the document. Since the C++ parser I am passing the XML to cannot read this, I want to avoid this character. Interestingly, when I create the writer like this</p>
  
  <pre><code>new XmlTextWriter(fs, null) 
  </code></pre>
  
  <p>I get the exact behaviour I expect. How do I rectreate this instantiation in code without leaving the parameter null?</p>