_get_field_default_gen does not return the correct generator when freezegun is in use

[markallanson]

The function _get_field_default_gen checks if a field's type is a datetime using t is datetime, which means if freezegun is being used in a test it generates the wrong default generator function.

Suggest this changes to t instanceof datetime to support cases like this.

[Gobot1234]

Fair enough, would you be willing to open a pull request to fix this?

[markallanson]

Yes! I am just heading out of the office on vacation and wanted to make sure I left at least a tombstone to create an PR for next week!