Encoding attribute from XML file Declaration turns to lower case when the file is created by a MemoryStream

Question

I have to generate a XML file using "ISO-8859-1" encoding from my Asp.Net Web API application but MemoryStream lowercases the encoding attribute from the generated XML definition to "iso-8859-1".

This method generates a XML file based on a object which has been created by a XSD.

 public static MemoryStream GenerateXml<T>(T entity) where T : class
 {
    XmlSerializerNamespaces ns = new XmlSerializerNamespaces();
    //Add an empty namespace and empty value
    ns.Add("", "");

    var memoryStream = new MemoryStream();
    var streamWriter = new StreamWriter(memoryStream, Encoding.GetEncoding("ISO-8859-1"));
    var serializer = new XmlSerializer(typeof(T));

    serializer.Serialize(streamWriter, entity, ns);

    return memoryStream;
 }

Then I need to use XDocument to replace the prefix definition of XML elements (Its a prerequisite that all elements should be only named with their own tags). So I had to do this:

 public MemoryStream GenerateXmlOpening<T>(T entity) where T : class
{
    var xmlMemStream = XmlHelper.GenerateXml(entity);

    xmlMemStream.Position = 0;
    XDocument doc = XDocument.Load(xmlMemStream, LoadOptions.PreserveWhitespace);
    //Removes the namespace declaration as prefix on elements
    doc.Descendants().Attributes().Where(a => a.IsNamespaceDeclaration).Remove();

    //the memory stream retreived from 'xmlMemStream' is already with "iso-8859-1 in lowercase, so im trying to override it
    doc.Declaration.Encoding = "ISO-8859-1";

    MemoryStream stream = new MemoryStream();

    // when i save the xdoc to the new memorystream, the encoding goes from "ISO-8859-1" to "iso-8859-1" again.
    doc.Save(stream);
    stream.Position = 0;

    return stream;
}

This is the beginning of the returned generated XML file:

<?xml version="1.0" encoding="iso-8859-1"?>
... content

How it's supposed to be:

<?xml version="1.0" encoding="ISO-8859-1"?>
... content

Ps.* Im writing the XML using a MemoryStream because I have to write a .zip file and return a response of all generated XML files within this zip. This .Zip generator receives a list of MemoryStreams.