Do not override widget class if already specified by user

[poplarShift]

Following the discussions in #1128, I discovered that providing the widgets dict currently did not allow specifying it for param.Number:

import param 
import panel as pn
pn.extension()

class T(param.Parameterized):
    a = param.Number(default=None)
t = T(a=1e-3)
    
pn.Pane(t.param, widgets={
    'a': pn.widgets.LiteralInput()
}) # falls back to Spinner no matter what

The reason was that param.Number was handled as a special case and actually overridden once again if no bounds are provided.

This PR simply adds another quick check and forces panel to roll with the user-supplied widgets, if provided, even for param.Number.

[philippjfr]

Thanks @poplarShift.