Improvement to wp-json api User check

[erwanlr]

Current check for users on the wp-json API rely on https://github.com/wpscanteam/wpscan/blob/master/app/finders/users/wp_json_api.rb#L56 for the URL. However, in some case, for example targets with a subdir, the wp-json URL may be incorrect (subdir/wp-json/wp/v2/users/ instead of wp-json/wp/v2/users/).

The homepage should be checked for the wp-json URL, if not found then the default should be used.

<link rel='https://api.w.org/' href='https://wp.lab/wp-json/' />
<link rel="EditURI" type="application/rsd+xml" title="RSD" href="https://wp.lab/cms/xmlrpc.php?rsd" />
<link rel="wlwmanifest" type="application/wlwmanifest+xml" href="https://wp.lab/cms/wp-includes/wlwmanifest.xml" />
<link rel='shortlink' href='https://wp.lab/' />
<link rel="alternate" type="application/json+oembed" href="https://wp.lab/wp-json/oembed/1.0/embed?url=https%3A%2F%2Fwp.lab%2F" />
<link rel="alternate" type="text/xml+oembed" href="https://wp.lab/wp-json/oembed/1.0/embed?url=https%3A%2F%2Fwp.lab%2F&#038;format=xml" />