Fix wrong Windows detection

Author: cosmo0920

src/main/java/org/embulk/output/CommandFileOutputPlugin.java

@@ -86,7 +86,7 @@ public List<String> get() {
 
         public ShellFactory build() {
             String osName = System.getProperty("os.name");
-            if(osName.contains("Windows")) {
+            if(osName.indexOf("Windows") >= 0) {
                 this.shell = ImmutableList.of("PowerShell.exe", "-Command");
             } else {
                 this.shell = ImmutableList.of("sh", "-c");

src/test/java/org/embulk/output/TestCommandFileOutputPlugin.java

@@ -29,7 +29,7 @@ public void testShell() {
         List<String> shell = shellFactory.get();
         String osName = System.getProperty("os.name");
         List<String> actualShellCmd;
-        if (osName.contains("Windows")) {
+        if (osName.indexOf("Windows") >= 0) {
             actualShellCmd = ImmutableList.of("PowerShell.exe", "-Command");
         } else {
             actualShellCmd = ImmutableList.of("sh", "-c");