Missing default type when using functionBuilder

**What happened**:
`functionBuilder.build()`  doesn't assign a default type to the returned function.

**What you expected to happen**:
The returned function should have the type `rest` is none is supplied

**How to reproduce it**:
```typescript
const fn = functionBuilder().build();
console.log(fn.type);
```

**Anything else we need to know?**:
Will be fixed in an upcoming PR
