__contains__ method for Headers class

slow/slow.py

@@ -33,6 +33,9 @@ class Headers:
     def __str__(self):
         return str(self._d)
 
+    def __contains__(self, key):
+        return self._d.__contains__(key)
+
 
 class Request:
     def __init__(self, method: str, path: str, headers: Headers, body: bytes):