Ensure a custom factory can be used with HttpClient#sendForm (#2265)

HttpClient#sendForm API uses BiConsumer and not BiFunction.
The user cannot change the encoder in the callback.
When a custom factory is provided, apply the changes when the control is again in Reactor Netty.

Fixes #2259

reactor-netty-http/src/main/java/reactor/netty/http/client/HttpClientFormEncoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2021 VMware, Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2011-2022 VMware, Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -131,7 +131,7 @@ public HttpClientForm factory(HttpDataFactory factory) {
 		}
 		this.newFactory = Objects.requireNonNull(factory, "factory");
 		this.needNewEncoder = true;
-		return applyChanges(request);
+		return this;
 	}
 
 	@Override
@@ -317,6 +317,8 @@ final HttpClientFormEncoder applyChanges(HttpRequest request) {
 
 			encoder.setBodyHttpDatas(getBodyListAttributes());
 
+			needNewEncoder = false;
+
 			return encoder;
 		}
 		catch (ErrorDataEncoderException ee) {

reactor-netty-http/src/test/java/reactor/netty/http/client/HttpClientWithTomcatTest.java

@@ -22,6 +22,7 @@
 import io.netty.handler.codec.http.HttpMethod;
 import io.netty.handler.codec.http.HttpRequest;
 import io.netty.handler.codec.http.HttpResponseStatus;
+import io.netty.handler.codec.http.multipart.DefaultHttpDataFactory;
 import io.netty.handler.codec.http.multipart.HttpData;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
@@ -109,6 +110,16 @@ void postUploadNoMultipart() throws Exception {
 		doTestPostUpload((req, form) -> form.multipart(false).attr("attr1", "attr2"), "attr1=attr2");
 	}
 
+	@Test
+	void postUploadNoMultipartWithCustomFactory() throws Exception {
+		doTestPostUpload((req, form) -> {
+			DefaultHttpDataFactory customFactory = new DefaultHttpDataFactory(DefaultHttpDataFactory.MINSIZE);
+			form.factory(customFactory)
+			    .multipart(false)
+			    .attr("attr1", "attr2");
+		}, "attr1=attr2");
+	}
+
 	@SuppressWarnings("unchecked")
 	private void doTestPostUpload(BiConsumer<? super HttpClientRequest, HttpClientForm> formCallback,
 			String expectedResponse) throws Exception {