Bug #1023

- Fixed bug #1023, which applied default.project and default.priority during
  modification (thanks to Christoph Lange).
- Added unit tests.
- Added Christoph to the AUTHORS file.

src/Task.h

@@ -102,7 +102,7 @@ public:
 
   void substitute (const std::string&, const std::string&, bool);
 
-  void validate ();
+  void validate (bool applyDefault = true);
 
   float urgency_c () const;
   float urgency ();