Combine cookies from original request and session file

Close #932
Co-authored-by: kbanc <katherine.bancoft@gmail.com>
Co-authored-by: Gabriel Cruz <gabs.oficial98@gmail.com>

httpie/sessions.py

@@ -4,6 +4,8 @@ Persistent, JSON-serialized sessions.
 """
 import os
 import re
+
+from http.cookies import SimpleCookie
 from pathlib import Path
 from typing import Iterable, Optional, Union
 from urllib.parse import urlsplit
@@ -76,7 +78,13 @@ class Session(BaseConfigDict):
                 continue  # Ignore explicitly unset headers
 
             value = value.decode('utf8')
-            if name == 'User-Agent' and value.startswith('HTTPie/'):
+            if name.lower() == 'user-agent' and value.startswith('HTTPie/'):
+                continue
+
+            if name.lower() == 'cookie':
+                for cookie_name, morsel in SimpleCookie(value).items():
+                    self['cookies'][cookie_name] = {'value': morsel.value}
+                del request_headers[name]
                 continue
 
             for prefix in SESSION_IGNORED_HEADER_PREFIXES: