fix '{…!j}' for otherwise non-serializable types (##2624)

like 'datetime'

gallery_dl/formatter.py

@@ -14,6 +14,7 @@ import string
 import _string
 import datetime
 import operator
+import functools
 from . import text, util
 
 _CACHE = {}
@@ -257,7 +258,7 @@ def parse_format_spec(format_spec, conversion):
             "u": str.upper,
             "c": str.capitalize,
             "C": string.capwords,
-            "j": json.dumps,
+            "j": functools.partial(json.dumps, default=str),
             "t": str.strip,
             "T": util.datetime_to_timestamp_string,
             "d": text.parse_timestamp,

test/test_formatter.py

@@ -56,6 +56,7 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{t!d:%Y-%m-%d}", "2010-01-01")
         self._run_test("{dt!T}", "1262304000")
         self._run_test("{l!j}", '["a", "b", "c"]')
+        self._run_test("{dt!j}", '"2010-01-01 00:00:00"')
 
         with self.assertRaises(KeyError):
             self._run_test("{a!q}", "hello world")