custom_schema not used on write_to

[braingram]

Description of the problem

Schema passed to custom_schema during AsdfFile.__init__ is not used for write_to.

Example of the problem

import asdf

schema_contents = """%YAML 1.1
---
id: "http://example.com/schemas/your-custom-schema"
$schema: "http://stsci.edu/schemas/yaml-schema/draft-01"
type: object
properties:
  value:
      type: integer"""


with open("my_schema.yaml", "w") as f:
    f.write(schema_contents)

af = asdf.AsdfFile(custom_schema="my_schema.yaml")
af["value"] = "a"
af.write_to("foo.asdf")  # no ValidationError

System information

asdf version: main
python version: 3.10
operating system: mac