Add support for DateField

Author: Sergi Pons Freixes

marshmallow_mongoengine/conversion/fields.py

@@ -163,6 +163,7 @@ class Builder(MetaFieldBuilder):
 register_field(me.fields.BinaryField, ma_fields.Raw)
 register_field(me.fields.BooleanField, ma_fields.Boolean)
 register_field(me.fields.ComplexDateTimeField, ma_fields.DateTime)
+register_field(me.fields.DateField, ma_fields.Date)
 register_field(me.fields.DateTimeField, ma_fields.DateTime)
 register_field(
     me.fields.DecimalField,