Fix `File#write_buffer` to always return the given `io`

Ref: ef89a62b70

This fixes a regression in 2.4.rc1.

Cherry-picked into 3.0 for consistency.

lib/zip/file.rb

@@ -301,7 +301,7 @@ module Zip
 
     # Write buffer write changes to buffer and return
     def write_buffer(io = ::StringIO.new)
-      return unless commit_required?
+      return io unless commit_required?
 
       ::Zip::OutputStream.write_buffer(io) do |zos|
         @cdir.each { |e| e.write_to_zip_output_stream(zos) }

test/file_test.rb

@@ -233,7 +233,7 @@ class ZipFileTest < MiniTest::Test
     assert zf.entries.map(&:name).include?('zippedruby1.rb')
 
     # Ensure the buffer isn't changed.
-    zf.write_buffer(string_io)
+    assert_same(string_io, zf.write_buffer(string_io))
     assert_equal(data, string_io.string)
   end