Fix: 'a' tag doesn't support sub-tags

[luzmane]

For tag <a type="note" l:href="#n1"><sup>1</sup></a> following exception will be thrown:

System.ArgumentNullException: The empty string '' is not a valid local name. (Parameter 'name')
   at System.Xml.XmlConvert.VerifyNCName(String name, ExceptionType exceptionType)
   at System.Xml.XmlConvert.VerifyNCName(String name)
   at System.Xml.Linq.XName..ctor(XNamespace ns, String localName)
   at System.Xml.Linq.XNamespace.GetName(String localName, Int32 index, Int32 count)
   at System.Xml.Linq.XNamespace.GetName(String localName)
   at System.Xml.Linq.XNamespace.op_Addition(XNamespace ns, String localName)
   at FB2Library.Elements.SimpleText.ToXML() in FB2Library\Elements\SimpleText.cs:line 218
   at FB2Library.Elements.InternalLinkItem.ToXML() in FB2Library\Elements\InternalLinkItem.cs:line 80
   at FB2Library.Elements.ParagraphItem.ToXML() in FB2Library\Elements\ParagraphItem.cs:line 180
   at FB2Library.Elements.SectionItem.ToXML() in FB2Library\Elements\SectionItem.cs:line 328
   at FB2Library.Elements.BodyItem.ToXML() in FB2Library\Elements\BodyItem.cs:line 164
   at FB2Library.FB2File.ToXML(Boolean bExportHeaderOnly) in FB2Library\FB2File.cs:line 420```