iso8601 format not detected correctly

[matejsp]

Date and DateTime with format argument as iso8601 it is not rendered correctly.

Actually marshallow supports also the following:

        "iso": utils.isoformat,
        "iso8601": utils.isoformat,
        "rfc": utils.rfcformat,
        "rfc822": utils.rfcformat,

works:
datetime = fields.DateTime(dump_only=True, format='iso')
does not work:
datetime2 = fields.DateTime(dump_only=True, format='iso8601')

https://github.com/marshmallow-code/marshmallow/blob/dev/src/marshmallow/fields.py#L1275-L1291

[sloria]

We haven't explicitly supported "iso8601" and "rfc822" (the docs only mention iso). But given that they're supported in marshmallow makes sense to support them here, too.

looks like the relevant check is here:

apispec/src/apispec/ext/marshmallow/field_converter.py

Line 571 in 9c3ac16

if field.format == "iso" or field.format is None:

PR welcome!