prt_shortpath() can return unexpected routing when `blend=TRUE'

[jmlondon]

@dsjohnson has been troubleshooting some unexpected routing behavior that first led us to issue #14. But, after testing a fix for that issue, the unexpected routing behavior persisted.

after further testing, we realized the unexpected routing was limited to situations where blend=TRUE. This functionality comes from the sfnetworks package but it isn't yet clear if the unexpected behavior is a bug in the sfnetworks::st_network_blend() function or something we're not handling correctly.