Support unicode data in emit

In xhr-polling context, using emit with unicode data will throw an encoding error :
```
UnicodeEncodeError: 'ascii' codec can't encode character u'\xc9' in position 126: ordinal not in range(128)
```

Multiple solutions :
# Change python requests (or other sublib) to make it force data encoding to utf-8
# Remove ensure_ascii from json.dumps and let json.dumps create ascii data
# Force packets encoding to utf-8 instead of unicode object

This problem does not exist with websocket transport because websocket-client force encoding to utf-8 when passed unicode objects.