warn with array should behave like puts with array

jruby 9.1.5.0 (2.3.1) 2016-09-07 036ce39 Java HotSpot(TM) 64-Bit Server VM 24.71-b01 on 1.7.0_71-b14 +jit [darwin-x86_64]
```
>> warn ["a", "b"]
["a", "b"]
=> nil
```

vs

```
 2.3.1
>> warn ["a", "b"]
a
b
```