add __hash__ method to Link object to be compatible with the __eq__ method

scrapy/link.py

@@ -18,6 +18,9 @@ class Link(object):
 
     def __eq__(self, other):
         return self.url == other.url and self.text == other.text
+    
+    def __hash__(self):
+        return hash(self.url) ^ hash(self.text)
 
     def __repr__(self):
         return '<Link url=%r text=%r >' % (self.url, self.text)