fix issue 538, currentZone in all hook methods will not be correct if…

[JiaLiPassion]

based on issue #538, all hook methods will pass wrong currentZone if the zonespec has no hook method but the parent Zone spec has.

the issue can be described as following case.

 var zone = Zone.current;
      var zoneA = zone.fork({
        name: 'A',
        onInvoke: function(parentDelegate, currentZone, targetZone, callback, applyThis, applyArgs, source) {
          expect(currentZone.name).toEqual('A');
          return parentDelegate.invoke(targetZone, callback, applyThis, applyArgs, source);
        }
      });
      var zoneB = zoneA.fork({
        name: 'B',
        onInvoke: function(parentDelegate, currentZone, targetZone, callback, applyThis, applyArgs, source) {
          expect(currentZone.name).toEqual('B');
          return parentDelegate.invoke(targetZone, callback, applyThis, applyArgs, source);
        }
      });
      var zoneC = zoneB.fork({
        name: 'C'
      });
      zoneC.run(function() {
      });

[mhevery]

Very nice work!