fix: dont add when helmetdata already has same instance

[king-of-emotion]

I use react-helmet-async like https://github.com/staylor/react-helmet-async#usage-without-context example, but every time I access the website that has Helmet Component, helmetData add same instance and not remove.

Dispatcher class has componentWillUnmount, but it is not called probably because this class render method return null.

So, I want to check the helmetData when add the instance that helmetData already has or not.

sorry for my poor english.

[king-of-emotion]

@staylor please check this pull request.

[gilluminate]

This will also address issues brought up in #213

NextJS apps will be able to use this package after this PR has been merged. Please bump this in priority.