Use provided request headers in urllib3

outside. However, `Urllib3HttpConnection` did not use these headers but only its default ones. With this commit, `Urllib3HttpConnection` will either use the default ones or merge the default headers with the provided ones if there are any. Relates elastic#618

Author: honzakral

elasticsearch/connection/http_urllib3.py

@@ -142,10 +142,11 @@ def perform_request(self, method, url, params=None, body=None, timeout=None, ign
  if not isinstance(method, str):
  method = method.encode('utf-8')
 
+ request_headers = self.headers
  if headers:
- request_headers = dict(self.headers)
- request_headers.update(headers or {})
- response = self.pool.urlopen(method, url, body, retries=False, headers=self.headers, **kw)
+ request_headers = request_headers.copy()
+ request_headers.update(headers)
+ response = self.pool.urlopen(method, url, body, retries=False, headers=request_headers, **kw)
  duration = time.time() - start
  raw_data = response.data.decode('utf-8')
  except Exception as e: