Update a String#{to_sym,intern} spec for "+,-"

Follow ticket-9048
See below links

* http://bugs.ruby-lang.org/issues/9048
* https://github.com/ruby/ruby/pull/383#issuecomment-26915103
Improve some special operators specs
Remove outdated special operators
Merge remote-tracking branch 'kachick/remove_special_operators' into ops

Conflicts:
	spec/ruby/core/string/shared/to_sym.rb