get_rest_url() incorrect returns the admin domain in switch_to_blog()

[cameronterry]

When called through rest_output_link_wp_head() (link) it returns the primary domain (correct). However, when called after using switch_to_blog(), it returns the admin domain (incorrect).

Discovered whilst adding the unit tests, which works with multisite through the switch_to_blog() / restore_current_blog() mechanism: #88.