CDRIVER-554: Inherit write concern in collection update/bulk

Closes #183

Author: jmikola

src/mongoc/mongoc-collection.c

@@ -1237,6 +1237,10 @@ mongoc_collection_update (mongoc_collection_t          *collection,
 
    bson_clear (&collection->gle);
 
+   if (!write_concern) {
+      write_concern = collection->write_concern;
+   }
+
    if (!((uint32_t)flags & MONGOC_UPDATE_NO_VALIDATE) &&
        bson_iter_init (&iter, update) &&
        bson_iter_next (&iter) &&
@@ -1799,6 +1803,10 @@ mongoc_collection_create_bulk_operation (
 {
    bson_return_val_if_fail (collection, NULL);
 
+   if (!write_concern) {
+      write_concern = collection->write_concern;
+   }
+
    /*
     * TODO: where should we discover if we can do new or old style bulk ops?
     */