Copy a PreparedRequest's CookieJar reliably

When a PreparedRequests's cookie jar is not a RequestsCookieJar instance, it
will not have a "copy" method. By adding _copy_cookie_jar we can reliably copy
cookie jars so that we have an actual copy instead of the same instance on
different prepared requests.

This also updates the RequestsCookieJar.update logic to create copies of
cookies from the other jar.

Closes #2527

requests/cookies.py

@@ -6,6 +6,7 @@ Compatibility code to be able to use `cookielib.CookieJar` with requests.
 requests.utils imports from here, so be careful with imports.
 """
 
+import copy
 import time
 import collections
 from .compat import cookielib, urlparse, urlunparse, Morsel
@@ -302,7 +303,7 @@ class RequestsCookieJar(cookielib.CookieJar, collections.MutableMapping):
         """Updates this jar with cookies from another CookieJar or dict-like"""
         if isinstance(other, cookielib.CookieJar):
             for cookie in other:
-                self.set_cookie(cookie)
+                self.set_cookie(copy.copy(cookie))
         else:
             super(RequestsCookieJar, self).update(other)
 
@@ -359,6 +360,21 @@ class RequestsCookieJar(cookielib.CookieJar, collections.MutableMapping):
         return new_cj
 
 
+def _copy_cookie_jar(jar):
+    if jar is None:
+        return None
+
+    if hasattr(jar, 'copy'):
+        # We're dealing with an instane of RequestsCookieJar
+        return jar.copy()
+    # We're dealing with a generic CookieJar instance
+    new_jar = copy.copy(jar)
+    new_jar.clear()
+    for cookie in jar:
+        new_jar.set_cookie(copy.copy(cookie))
+    return new_jar
+
+
 def create_cookie(name, value, **kwargs):
     """Make a cookie from underspecified parameters.
 

requests/models.py

@@ -15,7 +15,7 @@ from .hooks import default_hooks
 from .structures import CaseInsensitiveDict
 
 from .auth import HTTPBasicAuth
-from .cookies import cookiejar_from_dict, get_cookie_header
+from .cookies import cookiejar_from_dict, get_cookie_header, _copy_cookie_jar
 from .packages.urllib3.fields import RequestField
 from .packages.urllib3.filepost import encode_multipart_formdata
 from .packages.urllib3.util import parse_url
@@ -320,7 +320,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         p.method = self.method
         p.url = self.url
         p.headers = self.headers.copy() if self.headers is not None else None
-        p._cookies = self._cookies.copy() if self._cookies is not None else None
+        p._cookies = _copy_cookie_jar(self._cookies)
         p.body = self.body
         p.hooks = self.hooks
         return p