Fixed the lines that caused warnings when running in debug mode

Removes following warnings

lib/wasabi/core_ext/string.rb:8: warning: ambiguous first argument; put parentheses or even spaces
lib/wasabi/core_ext/string.rb:9: warning: ambiguous first argument; put parentheses or even spaces
lib/wasabi/core_ext/string.rb:10: warning: ambiguous first argument; put parentheses or even spaces
lib/wasabi/document.rb:138: warning: method redefined; discarding old xml