Allow blank for all string types (url, email, ...) on dumping too

allow_blank works only on load for fields.URL or fields.Email not dump:

from marshmallow import Schema, fields
class Test(Schema):
    url = fields.URL(allow_blank=True)

# '2.0.0-dev'
Test().dump({'url': ''})

returns: MarshalResult(data={u'url': None}, errors={u'url': [u'"" is not a valid URL.']})

I think it should return: MarshalResult(data={u'url': u''}, errors={})

[lucasvo]

I just found this PR: https://github.com/marshmallow-code/marshmallow/issues/167

But the change to remove allow_blank is missing from the changelog here: http://marshmallow.readthedocs.io/en/latest/changelog.html#changelog - would probably be helpful. (I was on 2.3 and used allow_blank extensively, this now just failed silently).

[deckar01]

Marshmallow 3.x no longer validates on dump, so the original issue no longer applies. https://github.com/marshmallow-code/marshmallow/issues/713 is a similar use case, and includes some examples that could work around this in marshmallow 2.x.

class OptionalUrl(fields.Url):
    def _serialize(self, value, attr, obj):
        if value == '':
            return ''
        return super()._serialize(value, attr, obj)