.rx.buffer initial value issue

I'm on param

```python
import param
rx_value = param.rx(1)
rx_buffer = rx_value.rx.buffer(3)

assert rx_buffer.rx.value==[1]

rx_value = 2
assert rx_buffer.rx.value==[1, 2]
```

```bash
Traceback (most recent call last):
File "/home/jovyan/repos/private/param/script.py", line 8, in
assert rx_buffer.rx.value==[1, 2]
^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError
```

I would expect the above example to work or alternatively the below example

```python
import param
rx_value = param.rx(1)
rx_buffer = rx_value.rx.buffer(3)

assert rx_buffer.rx.value==[]

rx_value = 2
assert rx_buffer.rx.value==[2]
```

```bash
Traceback (most recent call last):
File "/home/jovyan/repos/private/param/script.py", line 5, in
assert rx_buffer.rx.value==[]
^^^^^^^^^^^^^^^^^^^^^^
AssertionError
```

**I believe its not logical to have the initial value in the buffer but discard it when the next element is added.** Either it should not be included at all or it should be included until discarded by the buffer size.