Fix double slashes in HAL link urls

[rachelbaker]

HAL links and embedding are broken on sites with pretty permalinks as a result of #1426

[rachelbaker]

@rmccue This fixes the _embed issue if pretty permalinks are enabled.
BUT if permalinks are set to the default structure, query parameters don't work. As an example: http://example.com/wp-json/wp/v2/posts?posts_per_page=4 returns a 301 with a Location Header of http://example.com/wp-json/wp/v2/posts/?posts_per_page=4

Any suggestions?