Make Condition unit tests go through BinaryExpression

Instead of having Condition parse the left op right, make
BinaryExpression do it. Make sure all the tests pass as they
used to in the process.

lib/liquid/binary_expression.rb

@@ -30,6 +30,8 @@ module Liquid
         !equal_variables(left, right)
       when 'contains'
         contains(left, right)
+      else
+        raise(Liquid::ArgumentError, "Unknown operator #{operator}")
       end
     rescue ::ArgumentError => e
       raise Liquid::ArgumentError, e.message