Fixed pickle support for requests.adapters.HTTPAdapter by adding

'proxy_manager' when unpickling.

requests/adapters.py

@@ -97,6 +97,11 @@ class HTTPAdapter(BaseAdapter):
         self.init_poolmanager(self._pool_connections, self._pool_maxsize,
                               block=self._pool_block)
 
+        # Can't handle by adding 'proxy_manager' to self.__attrs__ because
+        # because self.poolmanager uses a lambda function, which isn't pickleable.
+        if not hasattr(self, 'proxy_manager'):
+            self.proxy_manager = {}
+
     def init_poolmanager(self, connections, maxsize, block=DEFAULT_POOLBLOCK):
         """Initializes a urllib3 PoolManager. This method should not be called
         from user code, and is only exposed for use when subclassing the