extra ' in ess comments

[jabranham]

Expected behavior

With | representing point in R-mode (from ESS):

## this is a test|

Pressing ' should then only insert one ' (it does this in emacs-lisp-mode, for example).

Actual behavior

It inserts 2 '

Steps to reproduce the problem

See above

Environment & version information

• smartparens version: 20170430.844
• Active major-mode: R-mode
• Emacs version (M-x emacs-version): 25.2.1
• OS: linux