Make Request#getExtra be generic.

webmagic-core/src/main/java/us/codecraft/webmagic/Request.java

@@ -78,14 +78,15 @@ public class Request implements Serializable {
         return this;
     }
 
-    public Object getExtra(String key) {
+    @SuppressWarnings("unchecked")
+    public <T> T getExtra(String key) {
         if (extras == null) {
             return null;
         }
-        return extras.get(key);
+        return (T) extras.get(key);
     }
 
-    public Request putExtra(String key, Object value) {
+    public <T> Request putExtra(String key, T value) {
         if (extras == null) {
             extras = new HashMap<String, Object>();
         }

webmagic-core/src/main/java/us/codecraft/webmagic/ResultItems.java

@@ -1,6 +1,5 @@
 package us.codecraft.webmagic;
 
-import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.Map;
 
@@ -21,6 +20,7 @@ public class ResultItems {
 
     private boolean skip;
 
+    @SuppressWarnings("unchecked")
     public <T> T get(String key) {
         Object o = fields.get(key);
         if (o == null) {