recurls: Switch response headers to HTTPMessage

CaseInsensitiveDict resulted with risky situations.

src/recurls/recurls.py

@@ -2,7 +2,6 @@ import json
 import urllib.parse
 import tempfile
 import subprocess
-from requests.structures import CaseInsensitiveDict
 import gzip
 import os
 import http.client
@@ -136,17 +135,14 @@ class Recurls(object):
         raise
       with open(response_header_file) as fh:
         response.header_text = fh.read()
-      response.headers = CaseInsensitiveDict()
+      response.headers = http.client.HTTPMessage()
       for line in response.header_text.splitlines()[1:]:
         if line.strip():
          if ':' not in line:
            continue
          header, value = line.split(':', 1)
          value = value.strip()
-         response.headers.setdefault(header, [])
-         response.headers[header].append(value)
-      for header in response.headers.keys():
-        response.headers[header] = ', '.join(response.headers[header])
+         response.headers.add_header(header, value)
       if response.headers.get('content-encoding') == 'gzip':
         with gzip.GzipFile(response_file) as fh:
           response.text = fh.read().decode()