[processing] support non-ogr layers in merge vector layers alg

(cherry picked from commit e338a90)

Author: nirvn

python/plugins/processing/algs/qgis/Merge.py

@@ -52,13 +52,12 @@ def defineCharacteristics(self):
 
     def processAlgorithm(self, progress):
         inLayers = self.getParameterValue(self.LAYERS)
-        paths = inLayers.split(';')
 
         layers = []
         fields = QgsFields()
         totalFeatureCount = 0
-        for x in xrange(0, len(paths)):
-            layer = QgsVectorLayer(paths[x], unicode(x), 'ogr')
+        for layerSource in inLayers.split(';'):
+            layer = dataobjects.getObjectFromUri(layerSource)
 
             if (len(layers) > 0):
                 if (layer.wkbType() != layers[0].wkbType()):