Ensure that static error pages have a Content-Type header

The getContentType() accessor method on View appears to only be used
for content negotiation. For the Content-Type header to be included in
the response the view needs to set it explicitly when render is called.

Closes gh-5918

Author: wilkinsona

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/DefaultErrorViewResolver.java

@@ -52,6 +52,7 @@
  * </ul>
  *
  * @author Phillip Webb
+ * @author Andy Wilkinson
  * @since 1.4.0
  */
 public class DefaultErrorViewResolver implements ErrorViewResolver, Ordered {
@@ -172,6 +173,7 @@ public String getContentType() {
 		@Override
 		public void render(Map<String, ?> model, HttpServletRequest request,
 				HttpServletResponse response) throws Exception {
+			response.setContentType(getContentType());
 			FileCopyUtils.copy(this.resource.getInputStream(),
 					response.getOutputStream());
 		}

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/DefaultErrorViewResolverTests.java

@@ -37,6 +37,7 @@
 import org.springframework.core.env.Environment;
 import org.springframework.core.io.ResourceLoader;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.web.servlet.ModelAndView;
@@ -53,6 +54,7 @@
  * Tests for {@link DefaultErrorViewResolver}.
  *
  * @author Phillip Webb
+ * @author Andy Wilkinson
  */
 public class DefaultErrorViewResolverTests {
 
@@ -145,23 +147,29 @@ public void resolveWhenExactResourceMatchShouldReturnResource() throws Exception
 		setResourceLocation("/exact");
 		ModelAndView resolved = this.resolver.resolveErrorView(this.request,
 				HttpStatus.NOT_FOUND, this.model);
-		assertThat(render(resolved)).isEqualTo("exact/404");
+		MockHttpServletResponse response = render(resolved);
+		assertThat(response.getContentAsString().trim()).isEqualTo("exact/404");
+		assertThat(response.getContentType()).isEqualTo(MediaType.TEXT_HTML_VALUE);
 	}
 
 	@Test
 	public void resolveWhenSeries4xxResourceMatchShouldReturnResource() throws Exception {
 		setResourceLocation("/4xx");
 		ModelAndView resolved = this.resolver.resolveErrorView(this.request,
 				HttpStatus.NOT_FOUND, this.model);
-		assertThat(render(resolved)).isEqualTo("4xx/4xx");
+		MockHttpServletResponse response = render(resolved);
+		assertThat(response.getContentAsString().trim()).isEqualTo("4xx/4xx");
+		assertThat(response.getContentType()).isEqualTo(MediaType.TEXT_HTML_VALUE);
 	}
 
 	@Test
 	public void resolveWhenSeries5xxResourceMatchShouldReturnResource() throws Exception {
 		setResourceLocation("/5xx");
 		ModelAndView resolved = this.resolver.resolveErrorView(this.request,
 				HttpStatus.INTERNAL_SERVER_ERROR, this.model);
-		assertThat(render(resolved)).isEqualTo("5xx/5xx");
+		MockHttpServletResponse response = render(resolved);
+		assertThat(response.getContentAsString().trim()).isEqualTo("5xx/5xx");
+		assertThat(response.getContentType()).isEqualTo(MediaType.TEXT_HTML_VALUE);
 	}
 
 	@Test
@@ -185,7 +193,9 @@ public void resolveWhenExactResourceMatchAndSeriesTemplateMatchShouldFavorResour
 				any(ResourceLoader.class))).willReturn(true);
 		ModelAndView resolved = this.resolver.resolveErrorView(this.request,
 				HttpStatus.NOT_FOUND, this.model);
-		assertThat(render(resolved)).isEqualTo("exact/404");
+		MockHttpServletResponse response = render(resolved);
+		assertThat(response.getContentAsString().trim()).isEqualTo("exact/404");
+		assertThat(response.getContentType()).isEqualTo(MediaType.TEXT_HTML_VALUE);
 	}
 
 	@Test
@@ -205,10 +215,10 @@ private void setResourceLocation(String path) {
 				"classpath:" + packageName.replace(".", "/") + path + "/" });
 	}
 
-	private String render(ModelAndView modelAndView) throws Exception {
+	private MockHttpServletResponse render(ModelAndView modelAndView) throws Exception {
 		MockHttpServletResponse response = new MockHttpServletResponse();
 		modelAndView.getView().render(this.model, this.request, response);
-		return response.getContentAsString().trim();
+		return response;
 	}
 
 }