Add the content_length field to the Document base class

tests/test_document_es4.py

@@ -272,3 +272,11 @@ def test_validate_unsigned_document(identity: Identity) -> None:
         document.validate_signature()
 
     assert str(ctx.value) == 'document has no signature assigned'
+
+
+def test_content_length() -> None:
+    """Test the content_length property"""
+
+    document = Es4Document.from_json(VALID_DOCUMENT)
+
+    assert document.content_length == 4