Add header accessors (#2696)

sanic/compat.py

@@ -88,6 +88,12 @@ class Header(CIMultiDict):
     very similar to a regular dictionary.
     """
 
+    def __getattr__(self, key: str) -> str:
+        if key.startswith("_"):
+            return self.__getattribute__(key)
+        key = key.rstrip("_").replace("_", "-")
+        return ",".join(self.getall(key, default=[]))
+
     def get_all(self, key: str):
         """
         Convenience method mapped to ``getall()``.