Add support for anything renderable as children rather than just strings.

[kaseyjcowley]

Hi all,

We recently upgraded to v5.0.3 and ran across the following error:

The error is due to only children of type string being supported rather than anything renderable. This PR contains a failing test with a example of using JS expressions within children.

From React docs https://facebook.github.io/react/docs/jsx-in-depth.html#children-in-jsx

In JSX expressions that contain both an opening tag and a closing tag, the content between those tags is passed as a special prop: props.children. There are several different ways to pass children [...]

Where children can typically be anything that can be rendered: numbers, strings, elements or an array (or fragment) containing these types (from PropTypes.children)

With the current API, I would expect children of these various types to be supported. However for example, <title> requires the children to be strings.

To summarize by examples:

const title = 'The Title';

// Works
<Helmet>
  <title>Title: The Title</title>
</Helmet>

// Works
<Helmet>
  <title>{'Title: The Title'}</title>
</Helmet>

// Works
<Helmet>
  <title>{'Title: ' + title}</title>
</Helmet>

// Doesn't Work
<Helmet>
  <title>Title: {title}</title>
</Helmet>

[CLAassistant]

All committers have signed the CLA.

[cwelch5]

@kchunterdeluxe Could you sign the CLA please? So we can give you credit for the unit test. Thanks!

[kaseyjcowley]

@cwelch5, CLA signed