use it on filesystem.py data sink

stix2/datastore/filesystem.py

@@ -13,7 +13,7 @@ from stix2.datastore import (
 )
 from stix2.datastore.filters import Filter, FilterSet, apply_common_filters
 from stix2.parsing import parse
-from stix2.serialization import serialize
+from stix2.serialization import fp_serialize
 from stix2.utils import format_datetime, get_type_from_id, parse_into_datetime
 
 
@@ -584,9 +584,8 @@ class FileSystemSink(DataSink):
         if os.path.isfile(file_path):
             raise DataSourceError("Attempted to overwrite file (!) at: {}".format(file_path))
 
-        with io.open(file_path, 'w', encoding=encoding) as f:
-            stix_obj = serialize(stix_obj, pretty=True, encoding=encoding, ensure_ascii=False)
-            f.write(stix_obj)
+        with io.open(file_path, mode='w', encoding=encoding) as f:
+            fp_serialize(stix_obj, f, pretty=True, encoding=encoding, ensure_ascii=False)
 
     def add(self, stix_data=None, version=None):
         """Add STIX objects to file directory.