Fix the issue of get Authorization header fails during bearer auth (#637)

Co-authored-by: yangben <yangben@zhihu.com>

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

@@ -34,8 +34,15 @@ class BearerAuthBackend(AuthenticationBackend):
         self.provider = provider
 
     async def authenticate(self, conn: HTTPConnection):
-        auth_header = conn.headers.get("Authorization")
-        if not auth_header or not auth_header.startswith("Bearer "):
+        auth_header = next(
+            (
+                conn.headers.get(key)
+                for key in conn.headers
+                if key.lower() == "authorization"
+            ),
+            None,
+        )
+        if not auth_header or not auth_header.lower().startswith("bearer "):
             return None
 
         token = auth_header[7:]  # Remove "Bearer " prefix