Make multi-line arrays always use trailing commas (#217)

This makes ArraysWithOneElementPerLine output arrays with commas after every element.

```
A = [1,2,3]
```

Now becomes:

```
A = [
  1,
  2,
  3,
]
```

marshal_test.go

@@ -775,7 +775,7 @@ func TestMarshalArrayOnePerLine(t *testing.T) {
   B = [
     1,
     2,
-    3
+    3,
   ]
   C = [1]
 `)