Fix behavior of ClientResponse#bodyTo** with Void

Prior to this commit, asking for a `Void` type using any of the
`ClientResponse#bodyTo*` methods would immediately return an empty
`Publisher` without consuming the response body.

Not doing so can lead to HTTP connection pool inconsistencies and/or
memory leaks, since:

* a connection that still has a response body being written to it cannot
be properly recycled in the connection pool
* incoming `DataBuffer` might not be released

This commit detects when `Void` types are asked as body types and in
those cases does the following:

1. Subscribe to the response body `Publisher` to allow the connection to
be returned to the connection pool
2. `cancel()` the body `Publisher` if the response body is not empty; in
that case, we choose to close the connection vs. consume the whole
response body

Those changes imply that `ClientHttpResponse` and other related
contracts don't need a `close()` method anymore.

Issue: SPR-16018

spring-web/src/main/java/org/springframework/http/client/reactive/ClientHttpResponse.java

@@ -16,8 +16,6 @@
 
 package org.springframework.http.client.reactive;
 
-import java.io.Closeable;
-
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ReactiveHttpInputMessage;
 import org.springframework.http.ResponseCookie;
@@ -30,7 +28,7 @@ import org.springframework.util.MultiValueMap;
  * @author Brian Clozel
  * @since 5.0
  */
-public interface ClientHttpResponse extends ReactiveHttpInputMessage, Closeable {
+public interface ClientHttpResponse extends ReactiveHttpInputMessage {
 
 	/**
 	 * Return the HTTP status as an {@link HttpStatus} enum value.
@@ -42,16 +40,4 @@ public interface ClientHttpResponse extends ReactiveHttpInputMessage, Closeable
 	 */
 	MultiValueMap<String, ResponseCookie> getCookies();
 
-	/**
-	 * Close this response and the underlying HTTP connection.
-	 * <p>This non-blocking method has to be called if its body isn't going
-	 * to be consumed. Not doing so might result in HTTP connection pool
-	 * inconsistencies or memory leaks.
-	 * <p>This shouldn't be called if the response body is read,
-	 * because it would prevent connections to be reused and cancel
-	 * the benefits of using a connection pooling.
-	 */
-	@Override
-	void close();
-
 }

spring-web/src/main/java/org/springframework/http/client/reactive/ClientHttpResponseDecorator.java

@@ -70,11 +70,6 @@ public class ClientHttpResponseDecorator implements ClientHttpResponse {
 		return this.delegate.getBody();
 	}
 
-	@Override
-	public void close() {
-		this.delegate.close();
-	}
-
 	@Override
 	public String toString() {
 		return getClass().getSimpleName() + " [delegate=" + getDelegate() + "]";

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpResponse.java

@@ -89,11 +89,6 @@ public class ReactorClientHttpResponse implements ClientHttpResponse {
 		return CollectionUtils.unmodifiableMultiValueMap(result);
 	}
 
-	@Override
-	public void close() {
-		this.response.dispose();
-	}
-
 	@Override
 	public String toString() {
 		return "ReactorClientHttpResponse{" +