Support customizing date format in model (de)serialization in Java client

and update the default date format to conform to RFC3339

modules/swagger-codegen/src/main/resources/Java/JSON.mustache

@@ -4,6 +4,8 @@ import com.fasterxml.jackson.annotation.*;
 import com.fasterxml.jackson.databind.*;
 import com.fasterxml.jackson.datatype.joda.*;
 
+import java.text.DateFormat;
+
 import java.io.IOException;
 
 {{>generatedAnnotation}}
@@ -20,6 +22,13 @@ public class JSON {
     mapper.registerModule(new JodaModule());
   }
 
+  /**
+   * Set the date format for JSON (de)serialization with Date properties.
+   */
+  public void setDateFormat(DateFormat dateFormat) {
+    mapper.setDateFormat(dateFormat);
+  }
+
   /**
    * Serialize the given Java object into JSON string.
    */