*of_check_with results in SchemaError due to unknown rule

[jasongraham]

Used Cerberus version / latest commit: 1.3

• I have the capacity to improve the docs when my problem is solved.

• I have the capacity to submit a patch when a bug is identified.

• My question does not concern a practical use-case that I can't figure out
  to solve.

---

Use-case abstract

I'm attempting to replace the usage of validators within my schemas with check_with.

I've run into a problem trying to use *of_check_with.

My previous usage of *of_validator did indeed work just fine, and I would like to continue using this pattern.

---

Bug report / Feature request

Attempting to use check-with in place of validator within an *of typesaver structure results in a SchemaError being raised.

Example contents of test.py

import cerberus

# Arbitrary dummy validators
def foo(field, value, error):
    ...

def bar(field, value, error):
    ...

schema = {
    "a": {
        # Also fails with oneof, allof, noneof
        # replacing with *of_validator works
        "anyof_check_with": [foo, bar]
    }
}

cerberus.Validator(schema)

Result: of running python test.py

Traceback (most recent call last):
  File "test.py", line 20, in <module>
    cerberus.Validator(schema)
  File .../python3.7/site-packages/cerberus/validator.py", line 181, in __init__
    self.schema = kwargs.get('schema', None)
  File ".../python3.7/site-packages/cerberus/validator.py", line 562, in schema
    self._schema = DefinitionSchema(self, schema)
  File ".../python3.7/site-packages/cerberus/schema.py", line 76, in __init__
    self.validate(schema)
  File ".../python3.7/site-packages/cerberus/schema.py", line 247, in validate
    self._validate(schema)
  File ".../python3.7/site-packages/cerberus/schema.py", line 263, in _validate
    raise SchemaError(self.schema_validator.errors)
cerberus.schema.SchemaError: {'a': [{'anyof_check_with': ['unknown rule']}]}

[funkyfuture]

thanks for reporting!