L.multipolyline doesn't exist

[schloerke]

Fixes: #512

R addPolylines() was broken when trying to use L.multiPolyline. Docs in: http://leafletjs.com/reference-1.3.0.html#polyline state L.polyline accepts an array of polyline objects, like what L.multiPolyline should accept. Changed the erroring function to L.polyline.

library(leaflet)
library(tigris)
#> To enable 
#> caching of data, set `options(tigris_use_cache = TRUE)` in your R script or .Rprofile.
#> 
#> Attaching package: 'tigris'
#> The following object is masked from 'package:graphics':
#> 
#>     plot
library(sp)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.1.3, proj.4 4.9.3

usa_states_sp = states(cb = TRUE, resolution = "20m")
#> 
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usa_states_sf = st_as_sf(usa_states_sp) %>% st_transform(crs = 4326)

leaflet() %>% addProviderTiles("Stamen.Toner") %>% addPolylines(data = usa_states_sp)

leaflet() %>% addProviderTiles("Stamen.Toner") %>% addPolylines(data = usa_states_sf)

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