Cannot invoke an expression whose type lacks a call signature (on function union)

tsc@2.5.3

```ts
// from lodash
declare function isFunction(func): func is ((...args: any[]) => any) | Function

if (isFunction(x)) {
  x('a') // TS2349
}
```

If I remove the union, it works:
```ts
declare function isFunction(func): func is ((...args: any[]) => any)

if (isFunction(x)) {
  x('a') // TS2349
}
```
