fix skippable_frame_spec.rb

Author: SpringMT

spec/zstd-skippable_frame_spec.rb

@@ -45,6 +45,9 @@
         convertible = Object.new
         convertible.define_singleton_method(:to_str) { frame }
         expect(Zstd.read_skippable_frame(convertible)).to eq "sample data"
+      end
+    end
+
     context 'large input (heap-allocated) + skippable frame' do
       it 'round-trips without breaking' do
         payload = 'A' * 1024