Fix inheritance of input method for prompt

Minibuffer inherits the input method from the current-buffer.  At the time `read-from-minibuffer` is called, the active buffer is the helm buffer displaynig candidates---this has always the default active method, thus the inheritance doesn't work.

We need to inherit the method from the `helm-current-buffer`, which is where the helm was called from.  We can set the method manually in the `minibuffer-setup-hook`