ob-shell: Fix wrong variable assignment binding

ob-shell: Fix wrong variable assignment binding

* lisp/ob-shell.el (org-babel-shell-initialize): Provide bindings to
  handle variable assignments for all the supported shells.
(org-babel--variable-assignments:sh-generic):
(org-babel--variable-assignments:bash_array):
(org-babel--variable-assignments:bash_assoc):
(org-babel--variable-assignments:bash): Rename using an internal
naming scheme since they are not meant to be called directly from
"ob-core.el".
(org-babel-variable-assignments:shell): Apply renaming.

Reported-by: David Dynerman <emperordali@block-party.net>
<http://permalink.gmane.org/gmane.emacs.orgmode/111139>