What is the server behavior for deserializing httpPrefixHeaders when multiple values appear for the same header name?

[david-perez]

The httpPrefixHeaders trait can be applied to structure members that target a map of string.

What map should a server implementation deserialize to when encountering multiple values for the same header name? The docs don't specify a behavior.

There are also no protocol tests.

Example (taken from here):

{
    id: "RestJsonHttpPrefixHeadersArePresent",
    documentation: "Adds headers by prefix",
    protocol: restJson1,
    method: "GET",
    uri: "/HttpPrefixHeaders",
    body: "",
    headers: {
        "X-Foo": "Foo",
        "X-Foo-Abc": "Abc value",
        "X-Foo-Abc": "another value",
        "X-Foo-Def": "Def value",
    },
    params: {
        foo: "Foo",
        fooMap: {
            Abc: "Abc value", // <------ Or `another value` ?
            Def: "Def value",
        }
    }
},

Should we accept the request and pick the first or the last value? Should we reject the request?