Adjust Posts Pagination Headers for filter Params

[rachelbaker]

Use the posts_per_page and paged values from $query_args instead of only from the shortened Request params when calculating the X-WP-TotalPages header as well as the prev and next link headers. This prevents incorrect headers when either the filter[posts_per_page] or filter[paged] request params are used, as well as hooks into the rest_post_query filter.

Fixes #1876

[rachelbaker]

@WP-API/amigos #reviewmerge

[rachelbaker]

Even though it looks like only the Posts endpoints use the filter param, the Users endpoint does include a filter (rest_user_query) so 5640d87 fixes the pagination headers there as well.