William Pitcock 689b0730f8 activitypub: fix date header format
HTTP date header specification says that days must always be two-digit.
Accordingly, change the format string used to ensure days are always
represented as two-digit (e.g. 01).
2019-03-01 12:22:45 +00:00
..
2018-12-31 15:41:47 +00:00
2019-02-09 16:31:17 +01:00
2019-02-20 16:51:25 +00:00
2019-02-22 13:29:52 +01:00