Early registration of ReadListener

This is similar to the WriteListener changes on the ServerHttpResponse
where we are more naturally exposed to a delayed write. Nevertheless
we could also have a delayed read and should be consistent. The early
initialization of the RequestBodyPublisher also simplifies the
internal implementation a bit.

spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpRequestTests.java

@@ -16,14 +16,18 @@
 
 package org.springframework.http.server.reactive;
 
+import java.io.ByteArrayInputStream;
 import java.util.Arrays;
 import java.util.Collections;
 import javax.servlet.AsyncContext;
+import javax.servlet.ReadListener;
+import javax.servlet.ServletInputStream;
 import javax.servlet.http.HttpServletRequest;
 
 import org.junit.Test;
 
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.mock.web.test.DelegatingServletInputStream;
 import org.springframework.mock.web.test.MockAsyncContext;
 import org.springframework.mock.web.test.MockHttpServletRequest;
 import org.springframework.mock.web.test.MockHttpServletResponse;
@@ -75,9 +79,25 @@ public class ServerHttpRequestTests {
 	}
 
 	private ServerHttpRequest createHttpRequest(String path) throws Exception {
-		HttpServletRequest request = new MockHttpServletRequest("GET", path);
+		HttpServletRequest request = new MockHttpServletRequest("GET", path) {
+			@Override
+			public ServletInputStream getInputStream() {
+				return new TestServletInputStream();
+			}
+		};
 		AsyncContext asyncContext = new MockAsyncContext(request, new MockHttpServletResponse());
 		return new ServletServerHttpRequest(request, asyncContext, new DefaultDataBufferFactory(), 1024);
 	}
 
+	private static class TestServletInputStream extends DelegatingServletInputStream {
+
+		public TestServletInputStream() {
+			super(new ByteArrayInputStream(new byte[0]));
+		}
+
+		@Override
+		public void setReadListener(ReadListener readListener) {
+			// Ignore
+		}
+	}
 }