ox: Fix "Wrong argument type" error when attributes start with :key ""

* lisp/ox.el (org-export-read-attribute): Fix "Wrong argument type"
  error when attributes start with :key "".
Merge branch 'maint'
org.el (org-reftex-citation): Fix a docstring

* lisp/org.el (org-reftex-citation): Fix contrib package name in the
docstring.

Thanks to ruediger on #org-mode for reporting this.
org-entities: Add support for hbar.

* lisp/org-entities.el (org-entities): Add support for hbar.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>