[Bug] MapServers are returned as a FeatureServer

[JosiahParry]

MapServers are conceptually different than a FeatureServer. Their class should reflect this. Reprex from #55

library(arcgis)
#> Attaching core arcgis packages:
#>   - {arcgisutils} v0.1.0
#>   - {arcgislayers} v0.1.0

nhd <- file.path(
  "https://hydro.nationalmap.gov", 
  "arcgis/rest/services", 
  "NHDPlus_HR", 
  "MapServer"
)

(nhd_srv <- arc_open(nhd))
#> <FeatureServer <13 layers, 0 tables>>
#> CRS: 3857
#> Capabilities: Data,Map,Query
#>   0: NHDPlusGage (esriGeometryPoint)
#>   1: NHDPlusSink (esriGeometryPoint)
#>   2: NHDPoint (esriGeometryPoint)
#>   3: NetworkNHDFlowline (esriGeometryPolyline)
#>   4: NonNetworkNHDFlowline (esriGeometryPolyline)
#>   5: FlowDirection (esriGeometryPolyline)
#>   6: NHDPlusWall (esriGeometryPolyline)
#>   7: NHDLine (esriGeometryPolyline)
#>   8: NHDArea (esriGeometryPolygon)
#>   9: NHDWaterbody (esriGeometryPolygon)
#>   10: NHDPlusCatchment (esriGeometryPolygon)
#>   11: NHDPlusBoundaryUnit (esriGeometryPolygon)
#>   12: WBDHU12 (esriGeometryPolygon)
class(nhd_srv)
#> [1] "FeatureServer"