Flush of underlying response in ContentCachingResponseWrapper

Prior to this commit, when adding a ShallowEtagHeaderFilter to an
application, the ServletResponse would be wrapped by a
ContentCachingResponseWrapper. When any part of the Spring
infrastructure calls `flushBuffer` on the wrapped response, the call is
delegated to the actual response, which is committed. It's not possible
to alter the response (headers, content) anymore - the ETag filter can't
act.

This change prevents the `flushBuffer` call to be delegated and only
commits the underlying response once the cached content is copied to the
actual response stream.

Issue: SPR-13717

spring-web/src/test/java/org/springframework/web/filter/ShallowEtagHeaderFilterTests.java

@@ -197,4 +197,25 @@ public class ShallowEtagHeaderFilterTests {
 		assertEquals("Invalid redirect URL", "http://www.google.com", response.getRedirectedUrl());
 	}
 
+	// SPR-13717
+	@Test
+	public void filterFlushResponse() throws Exception {
+		final MockHttpServletRequest request = new MockHttpServletRequest("GET", "/hotels");
+		MockHttpServletResponse response = new MockHttpServletResponse();
+
+		final byte[] responseBody = "Hello World".getBytes("UTF-8");
+		FilterChain filterChain = (filterRequest, filterResponse) -> {
+			assertEquals("Invalid request passed", request, filterRequest);
+			((HttpServletResponse) filterResponse).setStatus(HttpServletResponse.SC_OK);
+			FileCopyUtils.copy(responseBody, filterResponse.getOutputStream());
+			filterResponse.flushBuffer();
+		};
+		filter.doFilter(request, response, filterChain);
+
+		assertEquals("Invalid status", 200, response.getStatus());
+		assertEquals("Invalid ETag header", "\"0b10a8db164e0754105b7a99be72e3fe5\"", response.getHeader("ETag"));
+		assertTrue("Invalid Content-Length header", response.getContentLength() > 0);
+		assertArrayEquals("Invalid content", responseBody, response.getContentAsByteArray());
+	}
+
 }