Ensure response is committed after ErrorPageFilter

Otherwise Tomcat will go ahead and uncommit it and handle it again
in the ErrorReportValve (duh!)

Fixes gh-684

spring-boot/src/main/java/org/springframework/boot/context/web/ErrorPageFilter.java

@@ -91,6 +91,7 @@ class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer imple
 
 	private void doFilter(HttpServletRequest request, HttpServletResponse response,
 			FilterChain chain) throws IOException, ServletException {
+
 		ErrorWrapperResponse wrapped = new ErrorWrapperResponse(response);
 		try {
 			chain.doFilter(request, wrapped);
@@ -102,6 +103,7 @@ class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer imple
 		catch (Throwable ex) {
 			handleException(request, response, wrapped, ex);
 		}
+		response.flushBuffer();
 
 	}
 

spring-boot/src/test/java/org/springframework/boot/context/web/ErrorPageFilterTests.java

@@ -34,6 +34,7 @@ import org.springframework.mock.web.MockHttpServletResponse;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.junit.Assert.assertThat;
+import static org.junit.Assert.assertTrue;
 
 /**
  * Tests for {@link ErrorPageFilter}.
@@ -76,6 +77,7 @@ public class ErrorPageFilterTests {
 				equalTo((Object) 400));
 		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_MESSAGE),
 				equalTo((Object) "BAD"));
+		assertTrue(this.response.isCommitted());
 	}
 
 	@Test
@@ -96,6 +98,7 @@ public class ErrorPageFilterTests {
 				equalTo((Object) 400));
 		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_MESSAGE),
 				equalTo((Object) "BAD"));
+		assertTrue(this.response.isCommitted());
 	}
 
 	@Test
@@ -118,6 +121,7 @@ public class ErrorPageFilterTests {
 				equalTo((Object) "BAD"));
 		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_EXCEPTION_TYPE),
 				equalTo((Object) RuntimeException.class.getName()));
+		assertTrue(this.response.isCommitted());
 	}
 
 	@Test