Support single objects for meta prop

While passing meta values to <Helmet>, the provided API is an array of objects:

<Helmet
  meta={[
    { name: "description", content: "Helmet application" },
    { property: "og:type", content: "article" },
  ]}
/>

A co-worker recently tried passing a single object instead:

<Helmet
  meta={{ name: "description", content: "Helmet application" }}
/>

The consequences of this were rather severe; because we render on the server, React Helmet’s exception crashed a server crash. A careless deploy meant we spent quite a long time trying to figure out why our server kept going down.

Please understand that we accept responsibility for this, it was totally our fault for misunderstanding the documentation 😃

That said, it could be helpful to prevent similar mistakes by other users to support the ‘single object’ syntax, for when only a single meta property needs to be set.

Would be happy to look into this and open a PR, if it’s deemed a worthwhile inclusion!

Thanks.

[joshwcomeau]

Thanks @potench! #233 is awesome.

@jeffboyus I believe that they’ve since walked back the statement about ditching PropTypes. Unless there’s news that they’ve committed to removing them, I wouldn’t speculate.

[doctyper]

It’s a valid point, we should explore throwing a more descriptive error when users provide invalid values.

I believe we’re planning on adding and exporting Flow types to Helmet, which will also surface those errors to developers who use Flow types.