Refactor absence matcher

This is part of a collection of commits that aim to improve failure
messages across the board, in order to make matchers easier to debug
when something goes wrong.

* Make the description of the matcher more readable.
* Fix or fill in tests involving failure messages and descriptions to
  match recent changes to ValidationMatcher.

lib/shoulda/matchers/active_model/validate_absence_of_matcher.rb

@@ -78,20 +78,18 @@ module Shoulda
 
       # @private
       class ValidateAbsenceOfMatcher < ValidationMatcher
-        def with_message(message)
-          @expected_message = message
-          self
+        def initialize(attribute)
+          super
+          @expected_message = :present
         end
 
         def matches?(subject)
           super(subject)
-          @expected_message ||= :present
-
           disallows_value_of(value, @expected_message)
         end
 
-        def description
-          "require #{@attribute} to not be set"
+        def simple_description
+          "validate that :#{@attribute} is empty/falsy"
         end
 
         private
@@ -110,7 +108,7 @@ module Shoulda
             when :decimal then BigDecimal.new(1, 0)
             when :datetime, :time, :timestamp then Time.now
             when :date then Date.new
-            when :binary then "0"
+            when :binary then '0'
             else 'an arbitrary value'
             end
           end

spec/unit/shoulda/matchers/active_model/validate_absence_of_matcher_spec.rb

@@ -37,9 +37,20 @@ describe Shoulda::Matchers::ActiveModel::ValidateAbsenceOfMatcher, type: :model
     end
 
     context 'a model without an absence validation' do
-      it 'rejects' do
-        model = define_model(:example, attr: :string).new
-        expect(model).not_to validate_absence_of(:attr)
+      it 'rejects with the correct failure message' do
+        record = define_model(:example, attr: :string).new
+
+        message = <<-MESSAGE
+Example did not properly validate that :attr is empty/falsy.
+  After setting :attr to "an arbitrary value", the matcher expected the
+  Example to be invalid, but it was valid instead.
+        MESSAGE
+
+        assertion = lambda do
+          expect(record).to validate_absence_of(:attr)
+        end
+
+        expect(&assertion).to fail_with_message(message)
       end
     end
 
@@ -54,14 +65,18 @@ describe Shoulda::Matchers::ActiveModel::ValidateAbsenceOfMatcher, type: :model
     end
 
     context 'an ActiveModel class without an absence validation' do
-      it 'rejects' do
-        expect(active_model_with(:attr)).not_to validate_absence_of(:attr)
-      end
+      it 'rejects with the correct failure message' do
+        message = <<-MESSAGE
+Example did not properly validate that :attr is empty/falsy.
+  After setting :attr to "an arbitrary value", the matcher expected the
+  Example to be invalid, but it was valid instead.
+        MESSAGE
 
-      it 'provides the correct failure message' do
-        message = %{Expected errors to include "must be blank" when attr is set to "an arbitrary value",\ngot no errors}
+        assertion = lambda do
+           expect(active_model_with(:attr)).to validate_absence_of(:attr)
+        end
 
-        expect { expect(active_model_with(:attr)).to validate_absence_of(:attr) }.to fail_with_message(message)
+        expect(&assertion).to fail_with_message(message)
       end
     end
 
@@ -86,9 +101,20 @@ describe Shoulda::Matchers::ActiveModel::ValidateAbsenceOfMatcher, type: :model
     end
 
     context 'a non-absent has_and_belongs_to_many association' do
-      it 'rejects' do
+      it 'rejects with the correct failure message' do
         model = having_and_belonging_to_many(:children, absence: false)
-        expect(model).not_to validate_absence_of(:children)
+
+        message = <<-MESSAGE
+Parent did not properly validate that :children is empty/falsy.
+  After setting :children to [#<Child id: nil>], the matcher expected
+  the Parent to be invalid, but it was valid instead.
+        MESSAGE
+
+        assertion = lambda do
+          expect(model).to validate_absence_of(:children)
+        end
+
+        expect(&assertion).to fail_with_message(message)
       end
     end