Helmet.rewind might be undefined in Html.js

Hey,

in Html.js helper there is `const head = Helmet.rewind();` It might be undefined if there was no `` declared in current route. It will fail here (can't access property 'base' of undefined):

```
{head.base.toComponent()}
{head.title.toComponent()}
{head.meta.toComponent()}
{head.link.toComponent()}
{head.script.toComponent()}
```

I'm not sure if we can treat is as a bug because it works with current example setup (there is `` inside App.js). Maybe it should be checked for undefined and issue a warning to let know developer that there is no helmet declaration inside current route. Or maybe if it is undefined we should try to render it.

calling @sergesemashko as he provided PR for Helmet.