Default deserialization with Function field

Nobody has claimed this yet.

Based on the documentation for the deserialize parameter for `fields.Function`:

- deserialize – A callable from which to retrieve the value. The function must take a single argument value which is the value to be deserialized. It can also optionally take a context argument, which is a dictionary of context variables passed to the deserializer. If no callable is provided then `value` will be passed through unchanged.

I would expect that leaving the parameter out would populate the field with the source value, and that this code would work:

```
import marshmallow
from marshmallow import Schema, fields

class Bar(Schema):
baz = fields.String()
qux = fields.String()

class Foo(Schema):
baz = fields.Function(serialize=lambda x: x['bar']['baz'])
qux = fields.Function(serialize=lambda x: x['bar']['baz'])

@marshmallow.post_load
def post_load(self, data, many, **kwargs):
return {'bar': data}

s = {'baz': 'blue', 'qux': 'orange'}

print(s)
d = Foo().load(s)

print(d)
print(Foo().dumps(d))
```

However, I get the error:

`marshmallow.exceptions.ValidationError: {'baz': ['Unknown field.'], 'qux': ['Unknown field.']}`

It works as I would expect if I do

```
class Foo(Schema):
baz = fields.Function(serialize=lambda x: x['bar']['baz'],
deserialize=lambda x: x)
qux = fields.Function(serialize=lambda x: x['bar']['baz'],
deserialize=lambda x: x)
```

Am I just reading the documentation wrong, or is this a bug?

Start with the deserialization path for fields.Function and reproduce the shown Foo().load(s) example, comparing omitted deserialize with deserialize=lambda x: x. Done when the documented default behavior is either confirmed or corrected and the example loads without Unknown field errors while preserving the expected round trip.

Written by the indexing model from the issue text.