fix to proper echo of nvm which. Checkin test which had been left out

nvm.sh

@@ -968,7 +968,7 @@ nvm() {
 
       if [ "_$VERSION" = '_system' ]; then
         if nvm_has_system_node >/dev/null 2>&1; then
-          echo $(dirname `which node`)
+          echo $(nvm use system && echo dirname $(which node))
           return
         else
           echo "System version of node not found." >&2

test/fast/Listing paths/Running "nvm which foo" should return a nonzero exit code when not found

@@ -0,0 +1,6 @@
+ #!/bin/sh
+
+. ../../../nvm.sh
+
+nvm which nonexistent_version
+[ "$?" = "1" ]