GraphQlTester does not pass extensions for subscriptions

[rstoyanchev]

From #1131 (comment).

Extensions don't get included in WebSocket requests made with the GraphQlTester.

Minimal example:

@Test
fun testSubscription() {
    val client = StandardWebSocketClient()
    // spring-graphql instance running on port 61686
    val tester = WebSocketGraphQlTester
        .builder("ws://localhost:61686/graphql", client)
        .build()

    val query = "subscription TestSubscription(\$accessCode: String!) { testSubscription(accessCode: \$accessCode) }"
    val subscription = tester
        .document(query)
        .variable("accessCode", "testAccessCode")
        .extension("x-test-extension", "testExtension")
        .executeSubscription()
        .toFlux()

    StepVerifier
        .create(subscription)
        .expectNextMatches {
            it.path("testSubscription").entity(String::class.java).get() == ""
        }
        .thenCancel()
        .verify()
}

The test passes, but the server doesn't receive any extensions. It appears that this is caused by the AbstractGraphQlTesterBuilder which doesn't pass in extensions before executing its subscription.