fixes missing generics in response entity (#159)

Author: jmisur

spring-auto-restdocs-core/src/main/java/capital/scalable/restdocs/payload/AbstractJacksonFieldSnippet.java

@@ -87,7 +87,12 @@ protected Collection<FieldDescriptor> createFieldDescriptors(Operation operation
     }
 
     protected Type firstGenericType(MethodParameter param) {
-        return ((ParameterizedType) param.getGenericParameterType()).getActualTypeArguments()[0];
+        Type type = param.getGenericParameterType();
+        if (type != null && type instanceof ParameterizedType) {
+            return ((ParameterizedType) type).getActualTypeArguments()[0];
+        } else {
+            return Object.class;
+        }
     }
 
     protected abstract Type getType(HandlerMethod method);

spring-auto-restdocs-core/src/test/java/capital/scalable/restdocs/payload/JacksonResponseFieldSnippetTest.java

@@ -177,6 +177,20 @@ public void responseEntityResponse() throws Exception {
                 .build());
     }
 
+    @Test
+    public void responseEntityResponseWithoutGenerics() throws Exception {
+        HandlerMethod handlerMethod = createHandlerMethod("responseEntityItem2");
+
+        this.snippets.expect(RESPONSE_FIELDS).withContents(equalTo("No response body."));
+
+        new JacksonResponseFieldSnippet().document(operationBuilder
+                .attribute(HandlerMethod.class.getName(), handlerMethod)
+                .attribute(ObjectMapper.class.getName(), mapper)
+                .attribute(JavadocReader.class.getName(), javadocReader)
+                .attribute(ConstraintReader.class.getName(), constraintReader)
+                .build());
+    }
+
     @Test
     public void exactResponseType() throws Exception {
         HandlerMethod handlerMethod = createHandlerMethod("processItem");
@@ -299,6 +313,10 @@ public ResponseEntity<Item> responseEntityItem() {
             return ResponseEntity.ok(new Item("test"));
         }
 
+        public ResponseEntity responseEntityItem2() {
+            return ResponseEntity.ok(new Item("test"));
+        }
+
         public String processItem() {
             return "";
         }