You can set a custom body with a string, input stream or byte array. Also
add headers with a fluent builder API.

```java
MockResponse response = new MockResponse()
    .addHeader("Content-Type", "application/json; charset=utf-8")
    .addHeader("Cache-Control", "no-cache")
    .setBody("{}");
```

MockResponse can be used to simulate a slow network. This is useful for
testing timeouts and interactive testing.