The response is bytes plus a few named lines. The names are the whole contract, and the contract is short enough that writing it down is cheaper than remembering it.
About 1 min of reading
Write the header contract
Write the allowed headers
Before you run it, say out loud what it will print. Then open the answer.
Attempt an undeclared header
Before you run it, say out loud what it will print. Then open the answer.
Stamp the cached one
Before you run it, say out loud what it will print. Then open the answer.
The list is the contract of the route. A contract only in the code is a contract that drifts with each edit.
The one that names the request. A cached copy of it is a response about a request that is not the current one.
It says what the bytes are. A claim that disagrees with the bytes is a client that parses the wrong thing.
They say who may store the response. The policy is a decision about a future reader, not about the present one.
A client ignores what it does not know. Ignoring is why a wrong header is silent rather than loud.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.