adding check in layers to refresh heatmap data

In reference to issue 353 [https://github.com/tombatossals/angular-leaflet-directive/issues/353], @tombatossals you had mentioned heatmap data was static. Well, I noticed layers was already being watched, so I added a check to see if the layer is a heatmap type layer, and if so, call setData and update() on it. Now your heatmap can be as dynamic as your data changes and can still have multiple heatmap layers.