Deserialization crash in client when schema excludes fields

[brockhaywood]

In a response schema, if a nested field excludes fields from the schema that it nests, the swagger file generated by flask-rebar does not indicate that the excluded field will not be in the response.

Steps to reproduce

Define a schema similar to this

from marshmallow import fields
from flask_rebar.validation import ResponseSchema

class MyObjectSchema(ResponseSchema):
    my_uid = fields.UUID(required=True)
    my_field = fields.String(required=True)

class MyObjectListSchema(ResponseSchema):
    results = fields.Nested(
        MyObjectSchema,
        exclude=("my_field",),
        required=True,
        many=True,
    )

This will generate a swagger file with some of the following in it:

....
        "MyObjectSchema": {
            "properties": {
                "my_uid": {
                    "format": "uuid",
                    "type": "string"
                },
                "my_field": {
                    "type": "string"
                }
            },
            "required": [
                "my_uid",
                "my_field"
            ],
            "title": "MyObjectSchema",
            "type": "object"
        },
....
        "MyObjectListSchema": {
            "properties": {
                "results": {
                    "items": {
                        "$ref": "#/definitions/MyObjectSchema"
                    },
                    "type": "array"
                }
            },
            "required": [
                "results"
            ],
            "title": "MyObjectListSchema",
            "type": "object"
        },

So, what this results in, when deserializing objects in a swagger-codegen generated client is the following ValueError gets raised

@my_field.setter
    def my_field(self, my_field):
        if my_field is None:
>           raise ValueError("Invalid value for `my_field`, must not be `None`")
E           ValueError: Invalid value for `my_field`, must not be `None`