Issue with rnet_boundary points

[Robinlovelace]

As reported in #416, it was returning this:

boundary_points
#> Simple feature collection with 998 features and 0 fields
#> geometry type:  POINT
#> dimension:      XYZ
#> bbox:           xmin: 1 ymin: -0.116214 xmax: 499 ymax: -0.1021283
#> z_range:        zmin: 51.50323 zmax: 51.50875
#> geographic CRS: WGS 84
#> First 10 features:
#>                          geometry
#> 1  POINT Z (1 -0.113894 51.50631)
#> 2  POINT Z (1 -0.1127652 51.50...