Dummy User-Agent to appease Firefox and get t.co working.

It looks like Firefox will allow me to replace the User-Agent string,
but not for it to be empty. Twitter will offer a 301 Redirect (the
wanted result) to any random string.

src/background.js

@@ -54,7 +54,7 @@ function fetchNext (urls, respond, data) {
     let url = urls.shift()
     window.fetch(url, {
       method: 'HEAD',
-      headers: new Headers({ 'User-Agent': '' })
+      headers: new Headers({ 'User-Agent': 'verify-me-locally' })
     }).then(function (response) {
       if (response.url === data.to) {
         respond({status: 'verified', index: data.index})