Fix StringLiterals bug regarding character literals.

The StringLiterals cop shold not report offences for character literals.
Make more precise corrections in autocorrect.

Calling corrector.replace with rather big ranges increases the risk for clobbering, i.e. conflicting corrections. It's better to use insert_before, insert_after, and remove when possible, and replace only when it's relevant.
Fix double reporting of offence in SignalException.
Merge pull request #569 from jonas054/fix_autocorrect_clobbering

Fix autocorrect clobbering