Handled possible case of non-bytes result from self.formatted.format

[rmihael]

I stumbled upon the problem similar to vklochan/python-logstash#30 while using python-logstash-async with https://github.com/madzak/python-json-logger on Python 3.6:

Traceback (most recent call last):
  File "/Users/rmihael/Documents/Projects/trade/python-logstash-async/logstash_async/handler.py", line 68, in emit
    data = self._format_record(record)
  File "/Users/rmihael/Documents/Projects/trade/python-logstash-async/logstash_async/handler.py", line 127, in _format_record
    return self.formatter.format(record) + b'\n'
TypeError: must be str, not bytes

Returning text type instead of bytes from format seems normal to me. Handler appears to be a proper place to handle encoding.

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This change is 

[rmihael]

Hi! Have you had any chance to review this PR?

[eht16]

Not yet but will do in a few days. Stay tuned, sorry for the delay.

[eht16]

Looks good to me, thanks.

[rmihael]

Great! Will you make a release with this fix anytime soon?

[eht16]

Done: https://pypi.python.org/pypi/python-logstash-async/1.4.1