Type mismatch for date field

[markphilpot]

pelican.utils.get_date and the parser in general expects a string for the date field, but the frontmark reader returns a datetime.datetime object instead, ultimately causing a TypeError.

See METADATA_PROCESSORS in pelican.readers.py

[markphilpot]

This also happens if PyYaml interprets a field as a timestamp (like slug for example) where pelican's metadata processing assume it's going to receive a string...

[jlirochon]

As an example, if you set date: 2020-02-09T18:43:33.227Z in metadata, it will crash.

[jlirochon]

A workaround can be found here : jlirochon@e778136

Works for me, hope it can help someone.

Let me know if you want a PR or if it feels too hacky.