Enum Field() schema omits extra kwargs if no other overrides exist

[roxchkplusony]

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: 1.8.1
            pydantic compiled: True
                 install path: /Users/victor/git/data-mesh/module-specs/specs_common/.venv/lib/python3.8/site-packages/pydantic
               python version: 3.8.6 (default, Oct 10 2020, 00:41:34)  [Clang 12.0.0 (clang-1200.0.32.2)]
                     platform: macOS-10.15.7-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']

from pydantic import BaseModel, Field


class NumEnum(int, Enum):
    one = 1
    two = 2


class BugReproModel(BaseModel):
    num_enum: NumEnum = Field(extra_field='hello')


class WorkingModel(BaseModel):
    num_enum: NumEnum = Field(default=NumEnum.two, extra_field='hello')


def test_bug_repro_model():
    assert BugReproModel.schema()['properties']['num_enum'] == {
        '$ref': '#/definitions/NumEnum',
        'extra_field': 'hello', # this is missing
    }


def test_working_model():
    assert WorkingModel.schema()['properties']['num_enum'] == {
        'allOf': [{'$ref': '#/definitions/NumEnum'}],
        'default': NumEnum.two,
        'extra_field': 'hello', # this is here
    }

[PoncinMatthieu]

I encountered the same issue with enums. And of course the same thing happens if default value is None.
Our current workaround is to declare a Config class on the model and adding the field extra property by implementing the schema_extra method.
Not a regression as I was using pydantic 1.7.3. Upgrading to 1.8.2 didn't fix it.

[sammchardy]

Have added a PR to fix this issue.