code: def _merge_headers(self, call_specific_headers):
          
  
          # A case-insensitive mapping is necessary here so that there is
          # predictable behavior.  If a plain dict were used, you'd get keys in
          # the merged dict which differ only in case.  The requests library
          # would merge them internally, and it would be unpredictable which key
          # is chosen for the final set of headers.  Another possible approach
          # would be to upper/lower-case everything, but this seemed easier.  On
          # the other hand, I don't know if CaseInsensitiveDict is public API...?
  
          # First establish defaults
          merged_headers = requests.structures.CaseInsensitiveDict({
              "User-Agent": self.user_agent
          })
  
          # Then overlay with specifics from post/get methods
          if call_specific_headers:
              merged_headers.update(call_specific_headers)
  
          # Special "User-Agent" header check, to ensure one is always sent.
          # The call-specific overlay could have null'd out that header.
          if not merged_headers.get("User-Agent"):
              merged_headers["User-Agent"] = self.user_agent
  
          return merged_headers
docs: Merge headers from different sources together.  Headers passed to the
          post/get methods have highest priority, then headers associated with
          the connection object itself have next priority.
  
          :param call_specific_headers: A header dict from the get/post call, or
              None (the default for those methods).
          :return: A key-case-insensitive MutableMapping object which contains
              the merged headers.  (This doesn't actually return a dict.)