ox: Preserve coding system when writing output to a file

* lisp/ox.el (org-export-to-file): Preserve coding system when writing
  output.
Merge branch 'maint'
ox-ascii: Remove spurious new line between a table and the caption below

* lisp/ox-ascii.el (org-ascii-table): Remove spurious new line between
  a table and the caption below.

Thanks to Rick Frankel for reporting it and providing a patch.
Merge branch 'maint'
ox: Re-introduce `org-export-current-backend' for compatibility

* lisp/ox.el (org-export-current-backend): New variable.
(org-export-as): Use new variable.

Thanks to Christophe Rhodes for providing a preliminary patch.
Merge branch 'maint'
ox: Fix f5161671ceb882b7a946d0632fbbf5c380952556

* lisp/ox.el (org-export-as): Use new back-end structure.