Fixed the lines that caused warnings when running in debug mode

Removes following warnings:

lib/nori/core_ext/string.rb:8: warning: ambiguous first argument; put parentheses or even spaces
lib/nori/core_ext/string.rb:9: warning: ambiguous first argument; put parentheses or even spaces
lib/nori/core_ext/string.rb:10: warning: ambiguous first argument; put parentheses or even spaces