Use canonical content type header casing

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 4608fda7-552e-4297-a493-96bbeac57614

Author: andystaples

azure-functions-durable/azure/durable_functions/client.py

@@ -272,7 +272,7 @@ def create_check_status_response(
             body=str(payload),
             status_code=202,
             headers={
-                'content-type': 'application/json',
+                'Content-Type': 'application/json',
                 # Match v1: Location points at statusQueryGetUri, which includes
                 # the required query string (webhook key / task hub / connection)
                 # so a client that follows the header is authorized.
@@ -704,7 +704,7 @@ def create_check_status_response(
             body=str(payload),
             status_code=202,
             headers={
-                "content-type": "application/json",
+                "Content-Type": "application/json",
                 "Location": payload["statusQueryGetUri"],
                 "Retry-After": "10",
             },