Consider electric-pair-mode in tex-mode quotes

Fixes: debbugs:19356

* lisp/textmodes/tex-mode.el (tex-insert-quote): Consider and respect
`electric-pair-mode'.
Improve on previous commit for tex-mode quotes

Fixes: debbugs:19356

* tex-mode.el (tex-insert-quote): Consider and respect
`electric-pair-mode'.