text: Title: Date and DateTime with format argument not rendered correctly
  Body: When a Marshmallow `Date` or `DateTime` has a non-default `format` argument the output APISpec for the field has a `format: null` instead of `format: date` and `format: date-time` respectively.
  
  The expected behaviour should be the latter as it was happening for versions of `apispec` before 6.4.
  
  Created a draft PR https://github.com/marshmallow-code/apispec/pull/937 to showcase the fix.