Consistently use record.new_record?.

`be_a_new(model_class)` and `be_new_record` were
inconsistent in that one of them checked `!persisted?`
and one checked `new_record?`. The docs also said that
both checked `!persisted?` even only one did.

This changes both to use `new_record?` and to be documented
as such.

Fixes rspec#1801.

Author: myronmarston

lib/rspec/rails/matchers/be_a_new.rb

@@ -61,8 +61,8 @@ def unmatched_attributes
       end
 
       # @api public
-      # Passes if actual is an instance of `model_class` and returns `false` for
-      # `persisted?`. Typically used to specify instance variables assigned to
+      # Passes if actual is an instance of `model_class` and returns `true` for
+      # `new_record?`. Typically used to specify instance variables assigned to
       # views by controller actions
       #
       # Use the `with` method to specify the specific attributes to match on the

lib/rspec/rails/matchers/be_new_record.rb

@@ -4,7 +4,7 @@ module Matchers
       # @private
       class BeANewRecord < RSpec::Matchers::BuiltIn::BaseMatcher
         def matches?(actual)
-          !actual.persisted?
+          actual.new_record?
         end
 
         def failure_message
@@ -17,7 +17,7 @@ def failure_message_when_negated
       end
 
       # @api public
-      # Passes if actual returns `false` for `persisted?`.
+      # Passes if actual returns `true` for `new_record?`.
       #
       # @example
       #     get :new

spec/rspec/rails/matchers/be_new_record_spec.rb

@@ -1,8 +1,8 @@
 require "spec_helper"
 
 describe "be_new_record" do
-  context "un-persisted record" do
-    let(:record) { double('record', :persisted? => false) }
+  context "a new record" do
+    let(:record) { double('record', :new_record? => true) }
 
     it "passes" do
       expect(record).to be_new_record
@@ -15,8 +15,8 @@
     end
   end
 
-  context "persisted record" do
-    let(:record) { double('record', :persisted? => true) }
+  context "a persisted record" do
+    let(:record) { double('record', :new_record? => false) }
 
     it "fails" do
       expect(record).not_to be_new_record