More intuitive support for making Boolean's required.

validates_presence_of :field doesn't work for a Boolean field because false
is considered blank. Instead you have to use
validates_inclusions_of :field, :in => [true, false]. This is a known annoyance
of AR. We can make this better in MongoMapper by having the :required shortcut
do the right thing. So if you do:

    key :field, String, :required => true

Then it does a validates_presence_of :field. But if you do:

    key :flag, Boolean, :required => true

It does a validates_inclusion_of :field, :in => [true, false]

lib/mongo_mapper/plugins/keys.rb

@@ -121,7 +121,11 @@ def create_validations_for(key)
             attribute = key.name.to_sym
 
             if key.options[:required]
-              validates_presence_of(attribute)
+              if key.type == Boolean
+                validates_inclusion_of attribute, :in => [true, false]
+              else
+                validates_presence_of(attribute)
+              end
             end
 
             if key.options[:unique]

test/unit/test_validations.rb

@@ -245,6 +245,19 @@ class ValidationsTest < Test::Unit::TestCase
           doc.should_not have_error_on(:action)
         end
 
+        should "work with :required shortcut on Boolean type" do
+          @document.key :flag, Boolean, :required => true
+
+          doc = @document.new
+          doc.should have_error_on(:flag, 'is not included in the list')
+
+          doc.flag = true
+          doc.should_not have_error_on(:action)
+
+          doc.flag = false
+          doc.should_not have_error_on(:action)
+        end
+
         should "not have error if allow nil is true and value is nil" do
           @document.key :action, String
           @document.validates_inclusion_of :action, :in => %w(kick run), :allow_nil => true