Fix serializing readonly properties

spec/cr_serializer_spec.cr

@@ -736,5 +736,22 @@ describe CRS do
         p.class.should eq VirtualProperty
       end
     end
+
+    describe CRS::ReadOnly do
+      it "should include ReadOnly properties" do
+        properties = ReadOnly.new.serialization_properties
+        properties.size.should eq 2
+
+        p = properties[0]
+
+        p.name.should eq "name"
+        p.external_name.should eq "name"
+
+        p = properties[1]
+
+        p.name.should eq "password"
+        p.external_name.should eq "password"
+      end
+    end
   end
 end

spec/models/read_only.cr

@@ -1,8 +1,10 @@
 class ReadOnly
   include CrSerializer
 
-  property name : String
+  def initialize; end
+
+  property name : String = "name"
 
   @[CRS::ReadOnly]
-  property password : String?
+  property password : String? = nil
 end

src/CrSerializer.cr

@@ -211,7 +211,6 @@ module CrSerializer
             ivars = @type.instance_vars
               .reject { |ivar| ivar.annotation(CRS::Skip) }
               .reject { |ivar| ivar.annotation(CRS::IgnoreOnSerialize) }
-              .reject { |ivar| (ann = ivar.annotation(CRS::ReadOnly)); ann && !ivar.has_default_value? && !ivar.type.nilable? ? raise "#{@type}##{ivar.name} is read-only but is not nilable nor has a default value" : ann }
               .reject do |ivar|
                 not_exposed = (ann = @type.annotation(CRS::ExclusionPolicy)) && ann[0] == :all && !ivar.annotation(CRS::Expose)
                 excluded = (ann = @type.annotation(CRS::ExclusionPolicy)) && ann[0] == :none && ivar.annotation(CRS::Exclude)