Only do tracer logging if there are handlers

Fixes elastic#391, thanks cp2587 for the report!

Author: honzakral

elasticsearch/connection/base.py

@@ -65,7 +65,7 @@ def _pretty_json(data):
  logger.debug('> %s', body)
  logger.debug('< %s', response)
 
- if tracer.isEnabledFor(logging.INFO):
+ if tracer.isEnabledFor(logging.INFO) and tracer.hasHandlers():
  # include pretty in trace curls
  path = path.replace('?', '?pretty&', 1) if '?' in path else path + '?pretty'
  if self.url_prefix: