don't attempt to save AF::WithMetadata::MetadataNode#file_hash

`ActiveFedora::WithMetadata::MetadataNode` provides a `#file_hash` attribute,
but writing to it, even with the existing data, prevents all saves.

Wings needs to map data in and out of this class freely, and this attribute
functions as a honeypot. it baits Wings to preserve the data, and then fails
save when the converters try to do keep the existing data. since any attempt to
write this data will break save, ActiveFedora should treat it as read only.

never include `MetadataNode#file_hash` in changed attributes lists when Wings is
loaded.

lib/wings/setup.rb

@@ -48,6 +48,17 @@ def self.default_sort_params
     end
   end
 
+  module WithMetadata
+    class MetadataNode
+      ##
+      # @note fcrepo rejects `:file_hash` updates. the attribute is managed by
+      #   the data store. always drop it from changed attributes.
+      def changed_attributes
+        super.except(:file_hash)
+      end
+    end
+  end
+
   module Associations
     class ContainerProxy
       delegate :build_or_set, to: :@association

spec/models/hyrax/file_metadata_spec.rb

@@ -24,6 +24,24 @@
                           type: contain_exactly(described_class::Use::ORIGINAL_FILE))
   end
 
+  context 'when saved with a file' do
+    subject(:file_metadata) { Hyrax.custom_queries.find_file_metadata_by(id: file.id) }
+    let(:file_set) { FactoryBot.valkyrie_create(:hyrax_file_set) }
+
+    let(:file) do
+      Hyrax.storage_adapter.upload(resource: file_set,
+                                   file: Tempfile.new('blah'),
+                                   original_filename: 'blah.txt')
+    end
+
+    it 'can be changed and saved' do
+      file_metadata.creator = 'Tove'
+
+      expect(Hyrax.persister.save(resource: file_metadata).creator)
+        .to contain_exactly('Tove')
+    end
+  end
+
   describe '#original_file?' do
     context 'when use says file is the original file' do
       before do