Only shift if an argument is passed. Fixes #78.

libexec/rbenv-init

@@ -70,14 +70,17 @@ echo 'rbenv rehash 2>/dev/null'
 commands=(`rbenv commands --sh`)
 IFS="|"
 cat <<EOS
-function rbenv () {
-  command="\$1";
-  shift;
+function rbenv() {
+  command="\$1"
+  if [ "\$#" -gt 0 ]; then
+    shift
+  fi
+
   case "\$command" in
   ${commands[*]})
     eval \`rbenv "sh-\$command" "\$@"\`;;
   *)
     command rbenv "\$command" "\$@";;
-  esac;
-};
+  esac
+}
 EOS