title: Jackson ObjectMapper: readValue returns null
body: <p>I need to parse this json:</p>
  
  <pre><code>{
     "id":"cpd4-b39c4b2a-b5cb-4581-b519-6034aaa7fe4e",
     "transactionId":"768a9be4-b5b3-452f-9bd3-9fff2e9ace5c",
     "status":"PUBLIC",
     "confidential":true,
     "expiringAt":1231231,
     "locked":true,
     "metadata":[
        {
           "user":"admin",
           "creationTimestamp":1538578453285,
           "value":"metadata"
        }
     ],
     "security":"read",
     "timestampCreation":1538578453285,
     "userCreation":"admin",
     "appCreation":"app",
     "document":{
        "id":null,
        "transactionId":"768a9be4-b5b3-452f-9bd3-9fff2e9ace5c",
        "docId":"68aab3799a9380fe82ed43ff2d46a5b07da1b270-1282",
        "size":1282,
        "name":"pom.xml",
        "alias":"alias",
        "hash":"68aab3799a9380fe82ed43ff2d46a5b07da1b270",
        "title":"title",
        "encoding":"UTF-8",
        "mimeType":"application/xml"
     }
  }
  </code></pre>
  
  <p>to a object <code>Reference</code> class:</p>
  
  <pre><code>public class Reference {
  
      private String id;
      private String transactionId;
  
      private DocumentStatus status;
      private Boolean confidential;
      private Integer expiringAt;
      private Boolean locked;
  
      private List&lt;Metadata&gt; metadata;
      private String security;
  
      // IDReferenciaAlta
      private Date timestampCreation;
      private String userCreation;
      private String appCreation;
  
      private Date timestampModified;
      private String userModified;
      private String appModified;
  
      private Date timestampDeletion;
      private String userDeletion;
      private String appDeletion;
  
      //getters and setters...
  }
  </code></pre>
  
  <p>where <code>Metadata</code> is:</p>
  
  <pre><code>public class Metadata {
  
      private String user;
      private Date creationTimestamp;
      private String value;
  
      //getters an setters
  }
  </code></pre>
  
  <p>Currently, I'm using this code:</p>
  
  <pre><code>Reference reference = null;
  try {
      reference = this.mapper.readValue(jsonDocument, Reference.class);
  } catch (IOException e1) {
      // TODO: Throw domain exception...
      e1.printStackTrace();
  }
  </code></pre>
  
  <p>The problem is that <code>this.mapper.readValue(...)</code> returns <code>null</code>.</p>
  
  <p>I know that json schema and <code>Reference</code> class propoerties are not exactly the same, but I expected to get a reference with "common" json properties</p>