recurls: Enforce proper headers type

http.client.HTTPMessage is required for proper header setup.

src/recurls/recurls.py

@@ -5,6 +5,7 @@ import subprocess
 from requests.structures import CaseInsensitiveDict
 import gzip
 import os
+import http.client
 
 
 class RecurlsResponse(object):
@@ -88,7 +89,9 @@ class Recurls(object):
       elif isinstance(verify, str):
         command_list.extend(['--cacert', verify])
       if headers is None:
-        headers = {}
+        headers = http.client.HTTPMessage()
+      else:
+        assert isinstance(headers, http.client.HTTPMessage)
       with open(request_header_file, 'w') as fh:
         for header, value in headers.items():
           fh.write('%s: %s\n' % (header, value))