calendar.format: allow space or tabs in rfc3339

As per http://tools.ietf.org/html/rfc3339
```
NOTE: ISO 8601 defines date and time separated by "T".
Applications using this syntax may choose, for the sake of
readability, to specify a full-date and full-time separated by
(say) a space character.
```

YAML uses spaces or tabs (http://yaml.org/type/timestamp.html) so I added both.