Fix assume_init incorect usage

[kuznetsss]

Hi, thank you for this crate! It is a really good example how to create pty and spawn a command with it.
While implementing a similar thing in my project I've found a bug in assume_init() usage.
Currently some part of the buffer may be marked as initialised, while it was not actually used.
The implementation of assume_init() is:

    #[inline]
    pub unsafe fn assume_init(&mut self, n: usize) {
        let new = self.filled + n;
        if new > self.initialized {
            self.initialized = new;
        }
    }

Which means that it already counts previously filled bytes, so we don't need to provide prev_filled.

I removed prev_filled and added a test reproducing the bug.