Render all primitive types natively

If passed a String, Number, Boolean or Null value, `#serializer_for`
should understand that the value doesn't need a custom serializer with
all the bells and whistles.

lib/granola/helper.rb

@@ -35,14 +35,19 @@ def self.serializer_class_for(object)
     name = object.class.name
     Object.const_get("%sSerializer" % name)
   rescue NameError
-    const_get("%sSerializer" % name)
+    case object
+    when NilClass, TrueClass, FalseClass, Numeric, String
+      PrimitiveTypesSerializer
+    else
+      raise
+    end
   end
 
   # Internal: Null serializer that transparently handles rendering `nil` in case
   # it's passed.
-  class NilClassSerializer < Granola::Serializer
+  class PrimitiveTypesSerializer < Granola::Serializer
     def serialized
-      {}
+      object
     end
   end
 end

test/helper_test.rb

@@ -28,7 +28,7 @@ class ModelSerializer < Granola::Serializer
   assert_equal PersonSerializer, klass
 
   klass = Granola::Helper.serializer_class_for([])
-  assert_equal Granola::Helper::NilClassSerializer, klass
+  assert_equal Granola::Helper::PrimitiveTypesSerializer, klass
 
   klass = Granola::Helper.serializer_class_for(Namespaced::Model.new)
   assert_equal Namespaced::ModelSerializer, klass
@@ -53,7 +53,8 @@ class ModelSerializer < Granola::Serializer
 test "#serializer_for handles empty lists automatically" do
   serializer = serializer_for([])
   assert serializer.is_a?(Granola::List)
-  assert_equal Granola::Helper::NilClassSerializer, serializer.item_serializer
+  assert_equal \
+    Granola::Helper::PrimitiveTypesSerializer, serializer.item_serializer
 end
 
 test "#serializer_for accepts a Granola::Serializer" do |person|
@@ -67,3 +68,10 @@ class ModelSerializer < Granola::Serializer
   assert_equal expected, actual
   assert PersonSerializer === actual
 end
+
+test "#serializer_for correctly serializes primitive types" do
+  [nil, true, false, 10, 5.0, "foo"].each do |primitive_type|
+    serializer = serializer_for(primitive_type)
+    assert_equal primitive_type, serializer.serialized
+  end
+end