Fix #89, where - didn't trigger stdin parsing

acceptance.bats

@@ -12,6 +12,12 @@
   [ "$output" = "The document stdin contains a valid ReplicationController" ]
 }
 
+@test "Pass when parsing a valid Kubernetes config YAML file explicitly on stdin" {
+  run bash -c "cat fixtures/valid.yaml | kubeval -"
+  [ "$status" -eq 0 ]
+  [ "$output" = "The document stdin contains a valid ReplicationController" ]
+}
+
 @test "Pass when parsing a valid Kubernetes config JSON file" {
   run kubeval fixtures/valid.json
   [ "$status" -eq 0 ]

cmd/root.go

@@ -40,7 +40,8 @@ var RootCmd = &cobra.Command{
 			}
 		}
 		// We detect whether we have anything on stdin to process if we have no arguments
-		if len(args) < 1 && !windowsStdinIssue && ((stat.Mode() & os.ModeCharDevice) == 0) {
+		// or if the argument is a -
+		if (len(args) < 1 || args[0] == "-") && !windowsStdinIssue && ((stat.Mode() & os.ModeCharDevice) == 0) {
 			var buffer bytes.Buffer
 			scanner := bufio.NewScanner(os.Stdin)
 			for scanner.Scan() {