fix crs

[mpadge]

sf has deprecated proj4string (here and here), so osmdata needs to reflect this. Current test-sf-construction all fail because of this.

[mpadge]

That just fixes the tests. The main C++ routines still use a hard-coded proj4string, which must be deprecated. However, doing this without importing sf will require hard-coding the WKT version, which is perhaps not the best approach? Not important for the moment, as sf will still accept proj4string values for the foreseeable future, but this will need to be addressed at some stage.

[mpadge]

Needed to be done, as the proj4string deprecation notices form sf are now warnings which get propagated through to R CMD check results for packages using osmdata. That commit hard-codes the full WKT string, which seems to work seamlessly.