def _merge_headers(self, call_specific_headers):
<EOL>merged_headers = requests.structures.CaseInsensitiveDict({<EOL>"<STR_LIT>": self.user_agent<EOL>})<EOL>if call_specific_headers:<EOL><INDENT>merged_headers.update(call_specific_headers)<EOL><DEDENT>if not merged_headers.get("<STR_LIT>"):<EOL><INDENT>merged_headers["<STR_LIT>"] = self.user_agent<EOL><DEDENT>return merged_headers<EOL>
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.)