Change "object_modified" property of LocationContent to be

optional.  Add a corresponding unit test.

stix2/test/v21/test_language_content.py

@@ -71,3 +71,18 @@ def test_language_content_campaign():
     # or https://docs.python.org/2/library/json.html#json.dumps
     assert lc.serialize(pretty=True, ensure_ascii=False) == TEST_LANGUAGE_CONTENT
     assert lc.modified == camp.modified
+
+
+def test_object_modified_optional():
+    """
+    object_modified is now optional in STIX 2.1.
+    """
+
+    stix2.v21.LanguageContent(
+        object_ref=CAMPAIGN_ID,
+        contents={
+            "en": {
+                "name": "the english text",
+            },
+        },
+    )

stix2/v21/common.py

@@ -79,7 +79,7 @@ class LanguageContent(_STIXBase):
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('object_ref', ReferenceProperty(spec_version='2.1', required=True)),
         # TODO: 'object_modified' it MUST be an exact match for the modified time of the STIX Object (SRO or SDO) being referenced.
-        ('object_modified', TimestampProperty(required=True, precision='millisecond')),
+        ('object_modified', TimestampProperty(precision='millisecond')),
         # TODO: 'contents' https://docs.google.com/document/d/1ShNq4c3e1CkfANmD9O--mdZ5H0O_GLnjN28a_yrEaco/edit#heading=h.cfz5hcantmvx
         ('contents', DictionaryProperty(spec_version='2.1', required=True)),
         ('revoked', BooleanProperty()),