Update default character_types

[elimillera]

Per #65, dates and datetimes should be coerced to character.

[codecov]

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[thomas-neitmann]

@elimillera @zdz2101 I'd be great if you could merge this PR. "text" missing in the list of valid character types is a major pain point for participants of the admiral hackathon right now. I verified that after installing {xportr} from this branch columns of type "text" are no longer converted to numeric with values being set to NA.

[zdz2101]

Looks good to me!