[Feature Request] Autocomplete suggestions fallback

### Existing Component
Yes

### Component Name
el-autocomplete

### Description
Hi!

The suggestions require an array of objects and use the property `vale` as their content. Sometimes, we have just simple arrays.

Can we change:

```

{{ item[valueKey] }}

```

By?

```

{{ item[valueKey] === undefined ? item : item[valueKey] }}

```

It will make even easier to debug.

Thank you all!