Fix default graph translation in MemoryDatasource

Author: rubensworks

lib/datasources/MemoryDatasource.js

@@ -25,6 +25,8 @@ MemoryDatasource.prototype._getAllQuads = function (addQuad, done) {
 
 // Writes the results of the query to the given quad stream
 MemoryDatasource.prototype._executeQuery = function (query, destination) {
+  if (query.graph === '')
+    query.graph = this._quadStore.defaultGraph;
   var offset = query.offset || 0, limit = query.limit || Infinity,
       quads = this._quadStore.findByIRI(query.subject, query.predicate, query.object,
                                         query.graph);