Call accept_handler before append_to_history in Buffer.validate_and_handle

This was a regression from 6cfc3f3. Without this, any changes made to the buffer text in the accept handler are not reflected in the history. For instance, I call dedent(text).strip() in the accept handler.

Author: asmeurer

prompt_toolkit/buffer.py

@@ -1613,13 +1613,13 @@ def validate_and_handle(self):
 
  # When the validation succeeded, accept the input.
  if valid:
- self.append_to_history()
-
  if self.accept_handler:
  keep_text = self.accept_handler(self)
  else:
  keep_text = False
 
+ self.append_to_history()
+
  if not keep_text:
  self.reset()