use session's hooks as defaults for every requests.

requests/sessions.py

@@ -140,6 +140,10 @@ class Session(object):
         files = {} if files is None else files
         headers = {} if headers is None else headers
         params = {} if params is None else params
+        hooks = {} if hooks is None else hooks
+        # use session's hooks as defaults
+        for key, cb in self.hooks.iteritems():
+            hooks.setdefault(key, cb)
 
         # Expand header values.
         if headers: