bug fix for the PR #559 which was raised for issue #555

[sackh]

There was small bug in PR #559 if style is not present in input geojson properties, it fails with KeyError while updating style, Hence I added check for it. If style is present in input geojson properties then update the style provided by the user else assign the style.

e.g. from CountriesGeojson notebook

import ipyleaflet as ipyl
import ipywidgets as ipyw
import json
map = ipyl.Map(center=[53.88, 27.45], zoom=4)
label = ipyw.Label(layout=ipyw.Layout(width='100%'))

# geojson layer with hover handler
with open('./europe_110.geo.json') as f:
    data = json.load(f)
layer = ipyl.GeoJSON(data=data, hover_style={'fillColor': 'red'}, style = {'color': 'green', 'opacity':1, 'weight':1.9, 'dashArray':'9', 'fillOpacity':0.1})

This was failing with KeyError style
I have handled this in my changes.

@martinRenou - will you please review this.

[martinRenou]

Thanks a lot for fixing this issue and spotting it.

This code is getting out of hand it's becoming unreadable though, we might want to rewrite it a bit.