Fix the issue of get Authorization header fails during bearer auth

Author: yangben

src/mcp/server/auth/middleware/bearer_auth.py

@@ -34,7 +34,7 @@ def __init__(
         self.provider = provider
 
     async def authenticate(self, conn: HTTPConnection):
-        auth_header = conn.headers.get("Authorization")
+        auth_header = conn.headers.get("Authorization") or conn.headers.get("authorization")
         if not auth_header or not auth_header.startswith("Bearer "):
             return None