Bug with template OneOf()

Dear **Adrian**,

first of all I would like to say _Thank you_ for this awesome package.
It is really great.

Maybe I was not enough thorough but `'import_write': confuse.OneOf([bool, 'ask', 'skip']),` allows me to use such a values like `import_write: asdfasdfasdf` in `config_default.yaml`.

The example is the `example` of package so you can reproduce it by yourself.

What do I do wrongly?

Thanks for your help in advance.

Reproduce the report using the package example and the `config_default.yaml` entry `import_write: confuse.OneOf([bool, 'ask', 'skip'])`. Start by tracing how `OneOf()` validates that setting, then verify that `import_write: asdfasdfasdf` is rejected while the listed alternatives remain accepted.

Written by the indexing model from the issue text.