Assigning new label value to filedownload widget concatenates the value to previous value

[oggers]

Hi,

I'm just upgrading to panel 1.5.0, but this bug appeared in version 1.4.5 (1.4.4 was ok).

The problem is that when you update the value of the FileDownload label attribute with a different value as the previous one the value is concatenated to the previous value, but if you update it with the same value nothing happens.

To reproduce it use the following snippet (tested in jupyter):

import panel as pn

pn.extension()

download = pn.widgets.FileDownload()
download.label = 'Label 0'

def change_label_1(event):
    download.label = 'Label 1'

def change_label_2(event):
    download.label = 'Label 2'


button_1 = pn.widgets.Button(name='Change label 1')
button_1.on_click(change_label_1)

button_2 = pn.widgets.Button(name='Change label 2')
button_2.on_click(change_label_2)

pn.Column(download, button_1, button_2).show()```

After clicking [Change label 1] -> [Change label 1] -> [Change label 2] -> [Change label 1] -> [Change label 2] it shows:

Note that even if I clicked [Change label 1] two consecutive times it only shows up one time.

Thank you.