The tilde (~) character appears incorrectly serialized

To illustrate with a test case:

```
var o = new HashMap<String,Object>();
o.put("tilde", "~");
assertEquals("{\"tilde\":\"~\"}", JsonStream.serialize(new Config.Builder().escapeUnicode(false).build(), o));
```

```
org.junit.ComparisonFailure: 
Expected :{"tilde":"~"}
Actual   :{"tilde":"��"}
```