Fix JettyClientDecorator

Author: sarahchen6

dd-java-agent/instrumentation/jetty-client/jetty-client-9.1/src/main/java/datadog/trace/instrumentation/jetty_client91/JettyClientDecorator.java

@@ -3,6 +3,7 @@
 import datadog.trace.bootstrap.instrumentation.api.UTF8BytesString;
 import datadog.trace.bootstrap.instrumentation.decorator.HttpClientDecorator;
 import java.net.URI;
+import java.util.List;
 import org.eclipse.jetty.client.api.Request;
 import org.eclipse.jetty.client.api.Response;
 
@@ -38,7 +39,8 @@ protected int status(final Response httpResponse) {
 
   @Override
   protected String getRequestHeader(Request request, String headerName) {
-    return request.getHeaders().get(headerName);
+    List<String> result = request.getHeaders().getValuesList(headerName);
+    return String.join(",", result);
   }
 
   @Override