Add argument injection hook back

requests/sessions.py

@@ -157,6 +157,9 @@ class Session(object):
 
             args[attr] = merge_kwargs(local_val, session_val)
 
+        # Arguments manipulation hook.
+        args = dispatch_hook('args', hooks, args)
+
         r = Request(**args)
 
         # Don't send if asked nicely.
@@ -166,6 +169,7 @@ class Session(object):
         # Send the HTTP Request.
         r.send()
 
+
         return r.response