AspectPropertyValue reads the string "noValue" given as a value as no value #155

AspectPropertyValue.__init__ takes value = "noValue" as its default and sets a value only when value != "noValue" (aspectproperty_value.py lines 37 and 52 at rc cf904f8e). So a property whose real value is the text "noValue" reads as having no value, and value() raises. The default should be a private marker object that no caller can pass by accident. Changing it changes what a caller who passes "noValue" on purpose gets, so it needs a "Before you upgrade" note and is left out of the no value message draft (#134).