Use a case statement in Liquid::Parser#expression

lib/liquid/parser.rb

@@ -46,16 +46,14 @@ module Liquid
       tok[0] == type
     end
 
-    SINGLE_TOKEN_EXPRESSION_TYPES = [:string, :number].freeze
-    private_constant :SINGLE_TOKEN_EXPRESSION_TYPES
-
     def expression
       token = @tokens[@p]
-      if token[0] == :id
+      case token[0]
+      when :id
         variable_signature
-      elsif SINGLE_TOKEN_EXPRESSION_TYPES.include?(token[0])
+      when :string, :number
         consume
-      elsif token.first == :open_round
+      when :open_round
         consume
         first = expression
         consume(:dotdot)