Duplicate the context hash when duplicating an Attacher (#653)

Without this change, a single context hash is shared between the original attacher and its duplicate. The changes in fc6a3ed update this shared hash after duplication to reference the duplicate model, leading to both the duplicate and original attacher having `context[:record]` pointing at the duplicated model.

lib/shrine/attacher.rb

@@ -341,6 +341,13 @@ def shrine_class
 
       private
 
+      # The copy constructor that's called on #dup and #clone
+      # We need to duplicate the context to prevent it from being shared
+      def initialize_copy(other)
+        super
+        @context = @context.dup
+      end
+
       # Converts a String or Hash value into an UploadedFile object and ensures
       # it's uploaded to temporary storage.
       #

test/attacher_test.rb

@@ -6,6 +6,13 @@
     @shrine   = @attacher.shrine_class
   end
 
+  describe '#initialize_copy' do
+    it 'duplicates the context when duplicating the attacher' do
+      attacher_copy = @attacher.dup
+      refute_equal @attacher.context.object_id, attacher_copy.context.object_id
+    end
+  end
+
   describe ".from_data" do
     it "instantiates an attacher from file data" do
       file     = @attacher.upload(fakeio)

test/plugin/model_test.rb

@@ -136,6 +136,7 @@
         assert_equal model_copy, model_copy.file_attacher.record
         assert_equal :file, model_copy.file_attacher.name
         assert_equal Hash[record: model_copy, name: :file], model_copy.file_attacher.context
+        assert_equal Hash[record: model, name: :file], model.file_attacher.context
         assert model_copy.file_attacher.changed? # retains any state
       end