Use timezone marker in timestamp, fix extra bug.
Use timezone marker in timestamp, fix extra bug.

utcnow is not timezone aware so isoformat will not include a timestamp
marker. use strftime to create timestmap with timezone marker.
extra can be None when using logging.config.fileConfig, handle that.
Remove some extra shitespace.

exoscale/python-logstash-formatter,avisri/python-logstash-formatter,xgt001/python-logstash-formatter,shokunin/python-logstash-formatter,alphagov/python-logstash-formatter