Fix Feign ResponseEntity<Void> NPE

fixes gh-539

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/feign/support/ResponseEntityDecoder.java

@@ -39,8 +39,12 @@ public class ResponseEntityDecoder implements Decoder {
             type = ((ParameterizedType) type).getActualTypeArguments()[0];
             Object decodedObject = decoder.decode(response, type);
 
+            Class<?> clazz = null;
+            if (decodedObject != null) {
+                clazz = decodedObject.getClass();
+            }
             return createResponse(
-                    decodedObject.getClass(),
+                    clazz,
                     decodedObject,
                     response);
         }
@@ -56,9 +60,11 @@ public class ResponseEntityDecoder implements Decoder {
             headers.put(key, new LinkedList<>(response.headers().get(key)));
         }
 
-        return new ResponseEntity<T>(
-                clazz.cast(instance),
-                headers,
+        T retVal = null;
+        if (clazz != null && instance != null) {
+            retVal = clazz.cast(instance);
+        }
+        return new ResponseEntity<>(retVal, headers,
                 HttpStatus.valueOf(response.status()));
     }
 }

spring-cloud-netflix-core/src/test/java/org/springframework/cloud/netflix/feign/SpringDecoderTests.java

@@ -95,6 +95,17 @@ public class SpringDecoderTests extends FeignClientFactoryBean {
 		assertEquals("first hello didn't match", "hello world 1", hellos.get(0));
 	}
 
+	@Test
+	public void testResponseEntityVoid() {
+		ResponseEntity<Void> response = testClient().getHelloVoid();
+		assertNotNull("response was null", response);
+		List<String> headers = response.getHeaders().get("X-test-header");
+		assertNotNull("headers was null", headers);
+		assertEquals("headers size was wrong", 1, headers.size());
+		String header = headers.get(0);
+		assertEquals("header was wrong", "myval", header);
+	}
+
 	@Data
 	@AllArgsConstructor
 	@NoArgsConstructor
@@ -114,6 +125,9 @@ public class SpringDecoderTests extends FeignClientFactoryBean {
 
 		@RequestMapping(method = RequestMethod.GET, value = "/hellostrings")
 		public List<String> getHelloStrings();
+
+		@RequestMapping(method = RequestMethod.GET, value = "/hellovoid")
+		public ResponseEntity<Void> getHelloVoid();
 	}
 
 	@Configuration
@@ -126,6 +140,11 @@ public class SpringDecoderTests extends FeignClientFactoryBean {
 			return ResponseEntity.ok(new Hello("hello world via response"));
 		}
 
+		@Override
+		public ResponseEntity<Void> getHelloVoid() {
+			return ResponseEntity.noContent().header("X-test-header", "myval").build();
+		}
+
 		@Override
 		public Hello getHello() {
 			return new Hello("hello world 1");