`validate_arguments` does not assign `**kwargs` correctly

[antdking]

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: 1.7.3
            pydantic compiled: False
                 install path: /home/anthony/experiments/pydantic/pydantic
               python version: 3.8.5 (default, Aug 23 2020, 18:40:40)  [GCC 10.1.0]
                     platform: Linux-5.10.2-2-MANJARO-x86_64-with-glibc2.29
     optional deps. installed: ['devtools', 'dotenv', 'email-validator', 'typing-extensions']

from pydantic import validate_arguments

@validate_arguments
def foo(**kwargs):
    print(f"{kwargs=}")


def bar(**kwargs):
    print(f"{kwargs=}")

>>> foo(kwargs=1, b=2)
kwargs={'b': 2}
>>> bar(kwargs=1, b=2)
kwargs={'kwargs': 1, 'b': 2}

Kwargs doesn't get assigned as per native python implementation.

It looks like the field gets overwritten, as we're checking all input kwargs against all fields, including the special var_kwarg field.
https://github.com/samuelcolvin/pydantic/blob/8bad7bc91105616dfcc23ce1d68fba12f328a2a3/pydantic/decorator.py#L166

Should be as simple as set(self.model.__fields__) - {self.v_kwargs_name} to resolve, will prepare a patch now.