Document Basic auth header caching, guard against stale copies

client/src/main/java/org/asynchttpclient/Realm.java

@@ -249,7 +249,8 @@ public int getMaxIterationCount() {
 
     /**
      * Returns the lazily computed and cached HTTP Basic authorization header for this immutable realm.
-     * Concurrent first calls may compute the same value more than once.
+     * Always encodes {@link #getPrincipal()} and {@link #getPassword()} as a Basic header, regardless
+     * of {@link #getScheme()}. Concurrent first calls may compute the same value more than once.
      *
      * @return the Basic authorization header
      * @since 3.0.12

client/src/test/java/org/asynchttpclient/util/AuthenticatorUtilsTest.java

@@ -39,6 +39,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -66,6 +67,19 @@ void preemptiveBasicAuthorizationHeaderIsMemoizedOnRealm() {
         assertSame(first, proxy);
     }
 
+    @Test
+    void basicAuthHeaderCacheIsNotSharedAcrossRealms() {
+        Realm realmA = new Realm.Builder("user", "pass")
+                .setScheme(Realm.AuthScheme.BASIC)
+                .build();
+        Realm realmB = new Realm.Builder("user", "pass")
+                .setScheme(Realm.AuthScheme.BASIC)
+                .build();
+
+        assertEquals(realmA.getBasicAuthHeader(), realmB.getBasicAuthHeader());
+        assertNotSame(realmA.getBasicAuthHeader(), realmB.getBasicAuthHeader());
+    }
+
     @Test
     void computeBodyHashEmptyBody() throws Exception {
         Request request = new RequestBuilder("GET")