Bug fixes for REST API

[cameronterry]

Fixes two, separate, issues relating to the REST API. These are;

• No longer erroneously redirects REST API URLs using the admin domain.
  • REST API URLs on the mapped domain were unaffected.
• Changed to use "home_url" filter hook rather than "pre_option_home_url".
  • This is because "home_url" passes as a parameter the scheme, which is "rest" when called through get_rest_url(). This fixes issue #27.
  • This change / fix addresses REST API compatibility issues with plugins such as Redirection, Regenerate Thumbnails, and Yoast SEO.
  • The error in the browser console was; "Request header field x-wp-nonce is not allowed by Access-Control-Allow-Headers in preflight response."