only option not working properly for nested fields.

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```py
from marshmallow import Schema, fields
class Address:
def __init__(self, street, country):
self.street = street
self.country = country

class User:
def __init__(self, name, email, location):
self.name = name
self.email = email
self.location = location

class Blog:
def __init__(self, id, title, author):
self.title = title
self.id = id
self.author = author

class AddressSchema(Schema):
street = fields.String()
country = fields.String()

class UserSchema(Schema):
name = fields.String()
email = fields.Email()
location = fields.Nested(AddressSchema)

class BlogSchema(Schema):
id = fields.Int()
title = fields.String()
author = fields.Nested(UserSchema, only=['location'])

address = Address(street='4 cross', country='myCountry')
user = User(name="Anantha", email="anantha@python.org", location=address)
blog = Blog(id = 1, title="nested only test", author=user)

print(BlogSchema(only=['title','author.location.street']).dump(blog))
```
**Actual Result:** {'title': 'nested only test', 'author': {}}
**Expected Result:** {'title': 'nested only test', 'author': {'location': {'street': '4 cross'}}}
**Note:** if I change
```author = fields.Nested(UserSchema, only=['location'])``` to
```author = fields.Nested(UserSchema, only=['location.street'])``` it works.
But, This does not solve purpose as there may be many fields in location nested field and need different fields for different scenarios.

Start with the nested-field `only` handling exercised by `BlogSchema(only=['title','author.location.street']).dump(blog)` and reproduce the example in the issue. Trace how `author = fields.Nested(UserSchema, only=['location'])` is applied, then verify that the dump includes `author.location.street` while preserving the requested top-level fields.

Written by the indexing model from the issue text.