Support for ISO-8859-1

[microknights]

Today SoapCore only supports encoding of utf-8, utf-16 (+le/be), and not any legacy like ISO-8859-1.

So when starting the SoapCore consuming part like this:

            var soapEncoder = new SoapEncoderOptions
            { 
                WriteEncoding = Encoding.GetEncoding("ISO8859-1"),
                MessageVersion = MessageVersion.Soap11
            };
            app.UseSoapEndpoint<IMySoapConsumer>("/my/soap/consumer", soapEncoder,  SoapSerializer.XmlSerializer);

Will end in a exception, since XmlDictionaryReader.CreateTextReader only support the mentioned encodings.

This PR makes it so it falls back to XmlReader which has no encoding restrictions, and thereby support ISO-8859-1.

[kotovaleksandr]

Hi @microknights! Thanks for your contributing! Can you add unit tests for this change?