Use the standard idiom for null object for Durations

lib/cucumber/core/test/result.rb

@@ -200,15 +200,11 @@ class Duration
           def initialize(duration)
             @duration = duration
           end
-
-          def exist?
-            true
-          end
         end
 
         class UnknownDuration
-          def exist?
-            false
+          def nil?
+            true
           end
 
           def duration

spec/cucumber/core/test/duration_matcher.rb

@@ -5,13 +5,13 @@
 module Cucumber::Core::Test
   RSpec::Matchers.define :be_duration do |expected|
     match do |actual|
-      actual.exist? and actual.duration == expected
+      not actual.nil? and actual.duration == expected
     end
   end
 
   RSpec::Matchers.define :an_unknown_duration do
     match do |actual|
-      not actual.exist? and expect(actual).to respond_to(:duration)
+      actual.nil? and expect(actual).to respond_to(:duration)
     end
   end
 end

spec/cucumber/core/test/result_spec.rb

@@ -191,8 +191,8 @@ def describe_to(visitor, *args)
     describe Result::Duration do
       subject(:duration) { Result::Duration.new(10) }
 
-      it "exist? returns true" do
-        expect( duration.exist? ).to be_truthy
+      it "nil? returns false" do
+        expect( duration.nil? ).to be_falsy
       end
 
       it "has a duration" do
@@ -203,8 +203,8 @@ def describe_to(visitor, *args)
     describe Result::UnknownDuration do
       subject(:duration) { Result::UnknownDuration.new }
 
-      it "exist? returns false" do
-        expect( duration.exist? ).to be_falsy
+      it "nil? returns true" do
+        expect( duration.nil? ).to be_truthy
       end
 
       it "return duration 0" do