Change optimization task to not remove optional /Type entries contain…

…ing default values

An optional /Type field is currently removed like any other optional
field if it contains its default value.

Since this can lead to problems this change makes the optimization task
leave the /Type entries alone. For example, some Factur-X validators
need /Type /EmbeddedFile entries for embedded files because if this
entry is missing they declare the PDF non-conforming.

It is possible to work around this by not optimizing the file or
optimizing it and then adding the necessary keys back. But keeping the
/Type entry also has other benefits like automatic type wrapping in the
general case when such a file is loaded by HexaPDF again.

CHANGELOG.md

@@ -6,6 +6,11 @@
   a standard default appearance string for a variable text field if none is
   found
 
+### Changed
+
+* [HexaPDF::Task::Optimize] to not remove optional /Type entries containing
+  default values
+
 ### Fixed
 
 * [HexaPDF::Layout::TableBox] to correctly calculcate and distribute row

lib/hexapdf/task/optimize.rb

@@ -214,13 +214,13 @@ def self.process_xref_streams(doc, method)
         end
       end
 
-      # Deletes field entries of the object that are optional and currently set to their default
-      # value.
+      # Deletes field entries (except for /Type) of the object that are optional and currently set
+      # to their default value.
       def self.delete_fields_with_defaults(obj)
         return unless obj.kind_of?(HexaPDF::Dictionary) && !obj.null?
         obj.each do |name, value|
-          if (field = obj.class.field(name)) && !field.required? && field.default? &&
-              value == field.default
+          if name != :Type && (field = obj.class.field(name)) && !field.required? &&
+             field.default? && value == field.default
             obj.delete(name)
           end
         end

test/hexapdf/task/test_optimize.rb

@@ -8,6 +8,7 @@
   class TestType < HexaPDF::Dictionary
 
     define_type :Test
+    define_field :Type, type: Symbol, default: type
     define_field :Optional, type: Symbol, default: :Optional
 
   end
@@ -46,6 +47,7 @@ def assert_no_xrefstms
   end
 
   def assert_default_deleted
+    assert(@doc.object(1).key?(:Type))
     refute(@doc.object(1).key?(:Optional))
   end