google-protobuf under jruby has broken JSON encoding

encode_json.rb

# google-protobuf/master
require 'google/protobuf'
pool = Google::Protobuf::DescriptorPool.new
pool.build do
  add_message 'ProtoMsg' do
    repeated :msg_string,  :string,  1
  end
end
ProtoMsg  = pool.lookup('ProtoMsg').msgclass

pm = ProtoMsg.new
pm.msg_string += ['foo', 'bar']
pm #=> <ProtoMsg: msg_string: ["foo", "bar"]>
puts ProtoMsg.encode_json(pm) 
# {"msg_string":"#<Google::Protobuf::RepeatedField:0x1725dc0f>"}


# my patch
# ......
pm #=> <ProtoMsg: msg_string: ["foo", "bar"]>
puts ProtoMsg.encode_json(pm) 
# {"msg_string":["foo","bar"]}