Test Xml serialization of a property with attribute XmlText.
The tested property has a default value that is not null.
Covers 18908.
Property with attribute XmlTest is now set from ReadString only.
Before the change when the property default value was not null or empty the deserialized value would be the concatenation of the default value and the serialized value.
The correct behavior is to use the serialized value only.
Fixes 18908.
Merge pull request #1312 from esdrubal/xmltext

Property with attribute XmlTest is now set from ReadString only.