INT-4360: Add ClientStompEncoder support

The `StompSubProtocolHandler` explicitly sets `stompCommand` header
to the `MESSAGE` value ignoring any client inputs.
In this case the message is treated as from the server and ignored on
the STOMP Broker side from the client session.

* Introduce `ClientStompEncoder` for the client side to be injected
into the `StompSubProtocolHandler` for the proper client side messages
encoding/decoding.
Override `stompCommand` header to the `SEND` value if it is `MESSAGE`
before encoding to the `byte[]` to send to the session

JIRA: https://jira.spring.io/browse/INT-4360

**Cherry-pick to 4.3.x**

Fix WebSocket test to rely on the proper client config class
and don't pick up the server config unconditionally in the test context

spring-integration-websocket/src/main/java/org/springframework/integration/websocket/support/ClientStompEncoder.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.websocket.support;
+
+import java.util.Map;
+
+import org.springframework.messaging.simp.stomp.StompCommand;
+import org.springframework.messaging.simp.stomp.StompEncoder;
+import org.springframework.messaging.simp.stomp.StompHeaderAccessor;
+
+/**
+ * A {@link StompEncoder} extension to prepare a message for sending/receiving
+ * before/after encoding/decoding when used from WebSockets client side.
+ * For example it updates the {@code stompCommand} header from the {@code MESSAGE}
+ * to {@code SEND} frame, which is the case of
+ * {@link org.springframework.web.socket.messaging.StompSubProtocolHandler}.
+ *
+ * @author Artem Bilan
+ *
+ * @since 4.3.13
+ */
+public class ClientStompEncoder extends StompEncoder {
+
+	@Override
+	public byte[] encode(Map<String, Object> headers, byte[] payload) {
+		if (StompCommand.MESSAGE.equals(headers.get("stompCommand"))) {
+			StompHeaderAccessor stompHeaderAccessor = StompHeaderAccessor.create(StompCommand.SEND);
+			stompHeaderAccessor.copyHeadersIfAbsent(headers);
+			headers = stompHeaderAccessor.getMessageHeaders();
+		}
+		return super.encode(headers, payload);
+	}
+
+}