jooby-jackson 2.13.0 contains a breaking change when requiring ObjectMapper  
```java
package com.premiumminds.test;

import com.fasterxml.jackson.databind.ObjectMapper;
import io.jooby.Jooby;
import io.jooby.json.JacksonModule;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class App extends Jooby
{
    private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

    public static class MyObject {
        private final String message;

        public MyObject(final String message) {this.message = message;}

        public String getMessage() {
            return message;
        }
    }

    {
        install(new JacksonModule());

        get("/", ctx -> {

            MyObject myObject = new MyObject("Hello World");

            final ObjectMapper mapper = require(ObjectMapper.class);
            LOGGER.info(mapper.writeValueAsString(myObject));

            return myObject;
        });

    }

    public static void main(String[] args) {
        runApp(args, App::new);
    }
}
```

With jooby-jackson 2.12.0 
```shell
$ curl -H 'Content-Type: application/json' http://localhost:8080/ 
{"message":"Hello World"}
```
With jooby-jackson 2.13.0 
```shell
$ curl -H 'Content-Type: application/json' http://localhost:8080/ 
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<style>
body {font-family: "open sans",sans-serif; margin-left: 20px;}
h1 {font-weight: 300; line-height: 44px; margin: 25px 0 0 0;}
h2 {font-size: 16px;font-weight: 300; line-height: 44px; margin: 0;}
footer {font-weight: 300; line-height: 44px; margin-top: 10px;}
hr {background-color: #f7f7f9;}
div.trace {border:1px solid #e1e1e8; background-color: #f7f7f9;}
p {padding-left: 20px;}
p.tab {padding-left: 40px;}
</style>
<title>Server Error (500)</title>
<body>
<h1>Server Error</h1>
<hr>
<h2>message: Service not found: com.fasterxml.jackson.databind.ObjectMapper</h2>
<h2>status code: 500</h2>
</body>
</html>
```

This is related to this change https://github.com/jooby-project/jooby/commit/3189fd725b4b5fc58baa823d26667259ac361566#diff-3d2521b88af51d54e92d22f24cf540fe1f317562fd0847f7d3980b41a78da9bdR140 
It was registering `ObjectMapper` but now it is registering the mapper class which is `JsonMapper` by default. 
diff --git a/modules/jooby-jackson/src/main/java/io/jooby/json/JacksonModule.java b/modules/jooby-jackson/src/main/java/io/jooby/json/JacksonModule.java
index 42534163b..0a471050a 100644
--- a/modules/jooby-jackson/src/main/java/io/jooby/json/JacksonModule.java
+++ b/modules/jooby-jackson/src/main/java/io/jooby/json/JacksonModule.java
@@ -138,6 +138,7 @@ public class JacksonModule implements Extension, MessageDecoder, MessageEncoder
     ServiceRegistry services = application.getServices();
     Class mapperType = mapper.getClass();
     services.put(mapperType, mapper);
+    services.put(ObjectMapper.class, mapper);
 
     // Parsing exception as 400
     application.errorCode(JsonParseException.class, StatusCode.BAD_REQUEST);
['modules/jooby-jackson/src/main/java/io/jooby/json/JacksonModule.java']