BaseModel.copy(update=…) does not update __fields_set__

[yurikhan]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.7.2
            pydantic compiled: False
                 install path: /usr/local/lib/python3.6/dist-packages/pydantic
               python version: 3.6.9 (default, Oct  8 2020, 12:12:24)  [GCC 8.4.0]
                     platform: Linux-4.15.0-112-generic-x86_64-with-Ubuntu-18.04-bionic
     optional deps. installed: ['typing-extensions', 'email-validator']

from typing import Optional
import pydantic

class Foo(pydantic.BaseModel):
    foo: Optional[str]
    bar: Optional[str]

print(Foo(foo='hello')
      .copy(update={'bar': 'world'})
      .json(exclude_unset=True))
# Observed: {"foo": "hello"}
# Expected: {"foo": "hello", "bar": "world"}