Fix #dup for types which can refer to its owner by reference

Fixes #34.

generate.cr

@@ -572,14 +572,14 @@ class CClass < CNamespace
       end
     end
 
-    unless has_module?("NonCopyable") || has_module?("RenderTarget") || has_module?("AlResource") || module?
+    unless has_module?("NonCopyable") || has_module?("RenderTarget") || has_module?("AlResource") || module? || abstract?
       CFunction.new(@name.not_nil!, type: nil,
         parameters: [CParameter.new("copy", CType.new(self, reference: true, const: true))],
         visibility: Visibility::Public, parent: self, docs: [":nodoc:"]
       ).render(context, o)
       if context.crystal?
-        o<< "def dup() : self"
-        o<< "return typeof(self).new(self)"
+        o<< "def dup() : #{name(Context::Crystal)}"
+        o<< "return #{name(Context::Crystal)}.new(self)"
         o<< "end"
       end
     end

spec/view_spec.cr

@@ -0,0 +1,29 @@
+require "spec"
+require "../src/graphics"
+
+describe SF::View do
+  describe "#dup" do
+    it "works from a Reference (#34)" do
+      w = SF::RenderTexture.new
+      w.view = SF::View.new(SF.float_rect(1, 2, 3, 4))
+      v = w.view.dup
+      v.size.should eq SF.vector2(3, 4)
+    end
+
+    it "works as a class member (#34)" do
+      v = ViewDupTest.new.view_dup
+      v.size.should eq SF.vector2(3, 4)
+    end
+  end
+end
+
+class ViewDupTest
+  def initialize
+    @view = SF::View.new(SF.float_rect(1, 2, 3, 4))
+  end
+
+  def view_dup
+    @view.dup
+  end
+end
+