Fix not initalized instance variable warning

lib/capybara/selector/filters/expression_filter.rb

@@ -12,8 +12,11 @@ module Capybara
       end
 
       class IdentityExpressionFilter < ExpressionFilter
-        def initialize; end
+        def initialize(name)
+          super(name, nil, nil)
+        end
         def default?; false; end
+        def matcher?; false; end
         def apply_filter(expr, _name, _value); expr; end
       end
     end

lib/capybara/selector/selector.rb

@@ -223,7 +223,7 @@ module Capybara
     def xpath(*allowed_filters, &block)
       if block
         @format, @expression = :xpath, block
-        allowed_filters.flatten.each { |ef| expression_filters[ef] = Filters::IdentityExpressionFilter.new }
+        allowed_filters.flatten.each { |ef| expression_filters[ef] = Filters::IdentityExpressionFilter.new(ef) }
       end
       format == :xpath ? @expression : nil
     end