Only sets content type attr if not null.

fixes gh-374

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/NettyRoutingFilter.java

@@ -42,6 +42,7 @@ import org.springframework.web.server.ServerWebExchange;
 import static org.springframework.cloud.gateway.filter.headers.HttpHeadersFilter.filterRequest;
 import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR;
 import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR;
+import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.ORIGINAL_RESPONSE_CONTENT_TYPE_ATTR;
 import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.PRESERVE_HOST_HEADER_ATTRIBUTE;
 import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.isAlreadyRouted;
 import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.setAlreadyRouted;
@@ -114,7 +115,9 @@ public class NettyRoutingFilter implements GlobalFilter, Ordered {
 
 			res.responseHeaders().forEach(entry -> headers.add(entry.getKey(), entry.getValue()));
 
-			exchange.getAttributes().put("original_response_content_type", headers.getContentType());
+			if (headers.getContentType() != null) {
+				exchange.getAttributes().put(ORIGINAL_RESPONSE_CONTENT_TYPE_ATTR, headers.getContentType());
+			}
 
 			HttpHeaders filteredResponseHeaders = HttpHeadersFilter.filter(
 					this.headersFilters.getIfAvailable(), headers, exchange, Type.RESPONSE);

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/support/ServerWebExchangeUtils.java

@@ -48,6 +48,7 @@ public class ServerWebExchangeUtils {
 	public static final String GATEWAY_SCHEME_PREFIX_ATTR = qualify("gatewaySchemePrefix");
 	public static final String GATEWAY_PREDICATE_ROUTE_ATTR = qualify("gatewayPredicateRouteAttr");
 	public static final String WEIGHT_ATTR = qualify("routeWeight");
+	public static final String ORIGINAL_RESPONSE_CONTENT_TYPE_ATTR = "original_response_content_type";
 
 	/**
 	 * Used when a routing filter has been successfully call. Allows users to write custom

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/test/GatewayIntegrationTests.java

@@ -39,8 +39,11 @@ import org.springframework.context.annotation.Import;
 import org.springframework.core.annotation.Order;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringRunner;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.server.ServerWebExchange;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -146,13 +149,27 @@ public class GatewayIntegrationTests extends BaseWebClientTests {
 				.expectStatus().is5xxServerError();
 	}
 
+	@Test
+	// gh-374 no content type/empty body causes NPR in NettyRoutingFilter
+	public void noContentType() {
+		testClient.get().uri("/nocontenttype")
+				.exchange()
+				.expectStatus().is2xxSuccessful();
+	}
+
 	@EnableAutoConfiguration
 	@SpringBootConfiguration
 	@Import(DefaultTestConfig.class)
+	@RestController
 	public static class TestConfig {
 
 		private static final Log log = LogFactory.getLog(TestConfig.class);
 
+		@RequestMapping("/httpbin/nocontenttype")
+		public ResponseEntity<Void> nocontenttype() {
+			return ResponseEntity.status(204).build();
+		}
+
 		@Bean
 		@Order(-1)
 		public GlobalFilter postFilter() {