Add default in JSON Schema when const=True

[stur86]

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())":

python -c "import pydantic.utils; print(pydantic.utils.version_info())"
             pydantic version: 1.9.0
            pydantic compiled: True
                 install path: ...
               python version: 3.8.10 (tags/v3.8.10:3d8993a, May  3 2021, 11:48:03) [MSC v.1928 64 bit (AMD64)]
                     platform: Windows-10-10.0.22000-SP0
     optional deps. installed: ['typing-extensions']

When running this snippet of code:

from pydantic import BaseModel, Field

class MyModel(BaseModel):
    x: int = Field(default=0, const=True)

print(MyModel.schema_json(indent=2))

this is the result:

{
  "title": "MyModel",
  "type": "object",
  "properties": {
    "x": {
      "title": "X",
      "const": 0,
      "type": "integer"
    }
  }
}

I am having issues with some JSON Schema validator because of what I think is a subtle difference here - yes, we're setting x to be const=0, but we're not saying that the default is also zero, which leaves the field as not filled if one creates a default object. I don't think this is correct behaviour. I suggest deleting https://github.com/samuelcolvin/pydantic/blob/8997cc5961139dd2695761a33c06a66adbf1430a/pydantic/schema.py#L215 , which should fix the problem.

[samuelcolvin]

PR welcome, but you'll need to see if it breaks other tests and go back to where we originally added that line and see if there's a reason we did it that way.