fixes webpacker:clean erroring because of nested hashes on manifest

lib/webpacker/commands.rb

@@ -7,7 +7,7 @@ def initialize(webpacker)
 
   def clean(count_to_keep = 2)
     if config.public_output_path.exist? && config.public_manifest_path.exist?
-      files_in_manifest = manifest.refresh.values.map { |f| File.join config.root_path, "public", f }
+      files_in_manifest = process_manifest_hash(manifest.refresh)
       files_to_be_removed = files_in_manifest.flat_map do |file_in_manifest|
         file_prefix, file_ext = file_in_manifest.scan(/(.*)[0-9a-f]{20}(.*)/).first
         versions_of_file = Dir.glob("#{file_prefix}*#{file_ext}").grep(/#{file_prefix}[0-9a-f]{20}#{file_ext}/)
@@ -36,4 +36,13 @@ def compile
       manifest.refresh if success
     end
   end
+
+  private
+    def process_manifest_hash(manifest_hash)
+      manifest_hash.values.map do |value|
+        next process_manifest_hash(value) if value.is_a?(Hash)
+
+        File.join(config.root_path, "public", value)
+      end.flatten
+    end
 end

test/command_test.rb

@@ -24,4 +24,10 @@ def test_compile_command_returns_failure_status_when_stale
       end
     end
   end
+
+  def test_clean_command_works_with_nested_hashes_and_without_any_compiled_files
+    File.stub :delete, true do
+      assert Webpacker.commands.clean
+    end
+  end
 end