fix(layers): passing a geojson object should work

After ol3 v3.5 there has been a breaking change in how
the GeoJSON objects are treated. This commit fixes
the functionality to be able to pass an eagerly loaded
GeoJSON object and not just through the "url" prop.

Example:

```
var geoJsonLayer = {
  source: {
    type: 'GeoJSON',
    geojson: {
       object: {
          type: 'Feature',
          geometry: {
            type: 'Point',
            coordinates: [
              11.3243,
              46.508745
            ]
          }
       },
       projection: '...'
    }
  },
  style: {
     ...
  }
}

```

src/services/olHelpers.js

@@ -350,12 +350,17 @@ angular.module('openlayers-directive').factory('olHelpers', function($q, $log, $
                         url: source.url
                     });
                 } else {
-                    if (!isDefined(source.geojson.projection)) {
-                        source.geojson.projection = projection;
+                    oSource = new ol.source.Vector();
+
+                    var projectionToUse = projection;
+                    if (isDefined(source.geojson.projection)) {
+                        projectionToUse = source.geojson.projection;
                     }
-                    oSource = new ol.source.Vector(angular.extend(source.geojson, {
-                        format: new ol.format.GeoJSON()
-                    }));
+
+                    var geojsonFormat = new ol.format.GeoJSON();
+                    var features = geojsonFormat.readFeatures(source.geojson.object, { featureProjection: projectionToUse });
+
+                    oSource.addFeatures(features);
                 }
 
                 break;

test/unit/layersSpec.js

@@ -50,6 +50,44 @@ describe('Directive: openlayers layers', function() {
         expect(layers.item(1).getSource() instanceof ol.source.TileJSON).toBe(true);
     });
 
+    it('should properly render a GeoJSON layer containing the GeoJSON object', function(){
+        scope.geoJsonLayer = {
+            source: {
+                type: 'GeoJSON',
+                geojson: {
+                    object: {
+                        type: 'Feature',
+                        geometry: {
+                            type: 'Point',
+                            coordinates: [
+                                11.32436509382857,
+                                46.50874575613664
+                            ]
+                        }
+                    }
+                }
+            }
+        };
+
+        var element = angular.element('<openlayers>' +
+                                       '<ol-layer ol-layer-properties="geoJsonLayer"></ol-layer>' +
+                                       '</openlayers>');
+        element = $compile(element)(scope);
+
+        var layers;
+        olData.getMap().then(function(olMap) {
+            layers = olMap.getLayers();
+        });
+
+        scope.$digest();
+        expect(layers.item(0).getSource() instanceof ol.source.OSM).toBe(true);
+        expect(layers.getLength()).toBe(2);
+
+        var geoJsonLayer = layers.item(1);
+        expect(geoJsonLayer.getSource() instanceof ol.source.Vector).toBe(true);
+        expect(geoJsonLayer.getSource().getFeatures().length).not.toBe(0);
+    });
+
     it('should have one layer if custom-layers is used', function() {
         scope.mapbox = {
             source: {