Reenable ability of generators to return property-set as first item

Re-introduces functionality that was present in B2, but removed when the "" feature was introduced
Previously, generators could return a property-set as the first item in the result list, this feature removed that. It doesn't seem clear to me that removing this functionality was intentional or necessary to make the feature work. I suspect it was overlooked because the built-in generators did not utilize this functionality that the system supported

src/build/generators.jam

@@ -475,7 +475,13 @@ class generator
         }
         if $(result)
         {
-            return $(relevant) $(result) ;
+            if [ class.is-a $(result[1]) : property-set ]
+            {
+                return [ $(result[1]).add $(relevant) ] $(result[2-]) ;
+            }
+            else {
+               return $(relevant) $(result) ;
+            }
         }
     }