Allow for other http clients

I'd rather use Symfony's HttpClient than guzzle.  I believe there's a way to simply say "This needs a psr-7 http client, but it doesn't matter which one".  

Alas, I'm not sure of the details.  But since my project already uses the Symfony client, I don't want to introduce guzzle just for this one call.

Great library, BTW.  Thanks for releasing it.