Fix writing compressed requests

Tested with:

```
import performanceplatform.collector.write as write

config = {
  'url':'http://httpbin.org/post',
  'dry_run': False,
  'token': 'not-real',
}

big_string = "x" * 3000

data_set = write.DataSet.from_config(config)

data_set.post({'key': big_string})
```
Merge pull request #59 from alphagov/feature/compress-request-bodies

Fix writing compressed requests