Support of DateTime and Date formats

[expobrain]

Adds full support for rendering Date and DateTime Marshmallow fields with each of their format values.

This solve the issue of Date and DateTime with a non-default format being rendered with format: null instead of format: date and format: date-time respectively.

[lafrech]

Hi. Thanks for digging into this.

I didn't take to time to review thoroughly but here's a question already.

Any idea why it stopped working as expected in version 6.4?

[lafrech]

At this point, field.format can't be None due to line 549, right?

[bhperry]

Also, format and pattern should not ever by None, according to openapi spec they must be strings. Probably should be removed from the ret dict when they are None.