Test cases: 'unknownClass', 'nullClass' and 'classWithKnownAttributesAndWithoutConfiguration' for Instance. Adding the validation AttributeValue in InstanceHelper
 Fixing some problems on merge #43
Removing the name 'default' of exception messages of instance tests
creation of test instanceWithValidValuesForTheConfigurationOfAttributesText
Creation of test instanceWithValidValuesForTheConfigurationOfAttributesLongText
fixing the assertion default name for instance tests
Assigning a default value if available, an AttributeValue case is null