include date and date-time format in coreapi schema generation

[zbyte64]

This adds the proper format when translating serializers.DateField & DateTimeField to coreapi/coreschema

Helps down the line with swagger:
marcgibbons/django-rest-swagger#703

[carltongibson]

This seems reasonable yes.

@tomchristie Can I just ask you to comment on the format magic string values here? They seem reasonable but is there a canonical list of accepted values?

[zbyte64]

The format is from OpenAPI v2: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#data-types

Maybe also relevant; python-coreschema has format validating "email" & "uri": https://github.com/core-api/python-coreschema/blob/master/tests/test_string.py

[carltongibson]

@zbyte64 Good follow-up. Exactly right.

Give the TODO on CoreSchema we'll take this as-is for now and revisit (hopefully with named constants or such) when that gets tightened up.

Thanks for the input!