performance improvement pg_copy_to_string - change the way concat string

```ruby

require 'benchmark'

Benchmark.bm do |b| 
  b.report('+=') do
    data = ''
    line = 'hello word'

    100_000.times { data += line}
  end

  b.report('<<') do
    data = ''
    line = 'hello word'

    100_000.times { data << line}
  end

end
```

```
       user     system      total        real
+=  8.750000  13.420000  22.170000 ( 22.236511)
<<  0.010000   0.000000   0.010000 (  0.014012)
```