getallheaders() may not return array

[ericnicolaas]

I'm using this plugin on a SiteGround Cloud hosting account and seeing this warning in my logs every two minutes:

PHP Warning:  Invalid argument supplied for foreach() in /path/to/wordpress/wp-content/plugins/rest-api-oauth1/lib/class-wp-rest-oauth1.php on line 80

getallheaders may return FALSE on failure, which is what I assume is happening here. I'll submit a PR with a basic fix.

As to why getallheaders is returning false, I'm not sure. We have Varnish Cache on this site, and I know that affects cookies, so it could be related to that. We also have Cloudflare though I assume Cloudflare would play nice with headers.