Fixes #1536: asyncBuilder can override default logger (#1539)

Author: vitalijr2

core/src/main/java/feign/AsyncFeign.java

@@ -64,7 +64,7 @@ public static class AsyncBuilder<C> {
     private AsyncClient<C> client;
 
     private Logger.Level logLevel = Logger.Level.NONE;
-    private final Logger logger = new NoOpLogger();
+    private Logger logger = new NoOpLogger();
 
     private Decoder decoder = new Decoder.Default();
     private ErrorDecoder errorDecoder = new ErrorDecoder.Default();
@@ -176,6 +176,7 @@ public AsyncBuilder<C> contract(Contract contract) {
      */
     public AsyncBuilder<C> logger(Logger logger) {
       builder.logger(logger);
+      this.logger = logger;
       return this;
     }