Fix fortify-spatial namespace issue (#879)

[seancarmody]

Fixes fortify error when sp namespace is not loaded.
#879

[hadley]

In what scenario would sp() not be loaded?

[seancarmody]

Here's the example from the original issue:

bx <- rgeos::readWKT("POLYGON ((112.5 -44, 112.5 -9, 154 -9, 154 -44, + 112.5 -44))")
bx <- as(bx, "SpatialPolygonsDataFrame")
fortify(bx, region="dummy")
# Error in fortify.SpatialPolygonsDataFrame(bx, region = "dummy") : 
#  could not find function "polygons"

A more serious example is in the rmapaus package I have been working on, where I want to load map data and the use ggplot2 to plot it. I would prefer to stick to Imports rather than Depends, which I understand to be better practice.

[hadley]

Got it. Could you please add a short explanatory note to NEWS?

[seancarmody]

Done.

[lock]

This old issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with reprex) and link to this issue. https://reprex.tidyverse.org/