Fix setter for AbstractHandle.http_endpoint

Bug fixed

• http_endpoint property setter was renamed from set_http_endpoint. That's because this is the right, documented way to create a setter.

  # now, correct:
  instance = SomeInstance()
  old = instance.something
  instance.something = "new"
  
  # instead of wrong:
  instance = SomeInstance()
  old = instance.something
  instance.set_something = "new"