getHandle can be called multiple times.

We might not want to override existing reactive variable if there are
already dependencies on it.

Author: mitar

lib.coffee

@@ -9,7 +9,7 @@ class ComputedField
       Tracker.autorun (computation) ->
         value = func()
 
-        if computation.firstRun
+        unless lastValue
           lastValue = new ReactiveVar value, equalsFunc
         else
           lastValue.set value