Fixed "unknown command" on vanilla commands with no args (even though it's intended behavior...)

src/main/java/org/bukkit/command/defaults/OpCommand.java

@@ -37,6 +37,6 @@ public class OpCommand extends VanillaCommand {
 
     @Override
     public boolean matches(String input) {
-        return input.startsWith("op ");
+        return input.startsWith("op ") || input.equalsIgnoreCase("op");
     }
 }