Additional namespace declaration attributes in envelope

[ZokiPokiMk]

Added new option AdditionalEnvelopeXmlnsAttributes
when:
opt.AdditionalEnvelopeXmlnsAttributes = new Dictionary<string, string>() { {"arr","http://schemas.microsoft.com/2003/10/Serialization/Arrays" } };
writes xmlns:arr="http://schemas.microsoft.com/2003/10/Serialization/Arrays" in envelope:
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:arr="http://schemas.microsoft.com/2003/10/Serialization/Arrays">

[kotovaleksandr]

Hi @ZokiPokiMk Thanks for contribution! Please add unit tests and update readme.md