Changing == to self.equals in _propagate_value

[kefirbandi]

Hi,

it seems to me that in order to be consistent, we should not directly use '==' here, but self.equals.
At least this is how I got my code working.

[vidartf]

As the labels are supposed to be strings, I don't see what isn't working. Would you mind sharing a minimal example of code that doesn't currently work, but that this PR would fix?

[kefirbandi]

You're right, I messed up the code lines, now submitted a new version.
What I'm trying to fix is this code sample:

from ipywidgets.widgets import Dropdown
import pandas as pd
df1 = pd.DataFrame()
df2 = pd.DataFrame()
options = [['A',df1],['B',df2]]
dd=Dropdown(options=options, equals=lambda x,y : x is y)