Tests serialization of XmlNode field with attribute XmlAnyElement.
Covers #3211.
Test SerializationCodeGenerator.
XmlSerializerTests was mostly testing XmlSerializationReaderInterpreter and XmlSerializationWriterInterpreter.
XmlSerializerGeneratorTests forces the usage of SerializationCodeGenerator while running XmlSerializerTests.
Changed UsageCount and generationThreshold to zero based.
Added GetTypeFullName, fixes wrong nullable valuetype names.
Replaced incorrect bool.ToString() by GetLiteral(bool).
Added missing XmlTypeMapping.IsAny logic.
This complements commit 05444c7c1c570db730f17b6d15dd66d3728b6.
Fixed serialization of XmlNode field with attribute XmlAnyElement.
Fixes #3211.
Merge pull request #1223 from esdrubal/xmlgenerator

Test SerializationCodeGenerator.