Fix argument handling in main rbenv command

- Explicitly asking for help with `-h` or `--help` exits with 0 status
  and displays help on stdout.

- Not providing any arguments to rbenv results in failure status and
  displays version and help on stderr.

libexec/rbenv

@@ -91,12 +91,18 @@ shopt -u nullglob
 
 command="$1"
 case "$command" in
-"" | "-h" | "--help" )
-  echo -e "$(rbenv---version)\n$(rbenv-help)" >&2
+"" )
+  { rbenv---version
+    rbenv-help
+  } >&2
+  exit 1
   ;;
-"-v" )
+-v )
   exec rbenv---version
   ;;
+-h | --help )
+  exec rbenv-help
+  ;;
 * )
   command_path="$(command -v "rbenv-$command" || true)"
   if [ -z "$command_path" ]; then

test/rbenv.bats

@@ -4,7 +4,7 @@ load test_helper
 
 @test "blank invocation" {
   run rbenv
-  assert_success
+  assert_failure
   assert_line 0 "$(rbenv---version)"
 }