(PUP-3457) don't switch uid/gid if already the same

if the uid/gid is already the one we're supposed to switch to, do not
attempt to switch, just return.

fix a now failing #asuser rspec test, thanks @kylog!
add two rspec tests checking for the new functionality.
Merge pull request #3192 from igalic/no-suid-if-eq

(PUP-3457) don't switch uid/gid if already the same