ajv-keywords typeof function and default

**What version of Ajv are you using? Does the issue happen if you use the latest version?**

6.12.2

**Ajv options object**

```javascript
{ useDefaults: true }
```

**JSON Schema**

```js
{
"type": "object",
"properties": {
"foo": { "typeof": "function", "default": () => {} }
}
}
```

**Sample data**

See below.

**Your code**

https://runkit.com/satazor/5eac34960c2691001afd90ab

**Validation result, data AFTER validation, error messages**

Throws:

> should pass \"typeof\" keyword validation

**What results did you expect?**

Because the `foo` prop has a default set to `() => {}`, it should not throw.
The **issue** is that `ajv` is doing `JSON.stringify` on the default value, to clone values. However, that should not be done for functions, and possible other types.

~~Using `{ useDefaults: 'shared' }` fixes the issue, but it does introduce other problematic side-effects.~~ Actually, there's a missing `function` entry here: https://github.com/epoberezkin/ajv/blob/3b735371e74bf847146e07b59172a6d4485cc44a/lib/compile/index.js#L238

**Are you going to resolve the issue?**

I may try.