add tests for GreetingController

src/main/kotlin/com/example/demo/controllers/GreetingController.kt

@@ -2,6 +2,7 @@ package com.example.demo.controllers
 
 import com.example.demo.provider.html.renderProductTable
 import org.springframework.web.bind.annotation.GetMapping
+import org.springframework.web.bind.annotation.ResponseBody
 import org.springframework.web.bind.annotation.RestController
 
 @RestController
@@ -11,7 +12,8 @@ class GreetingController {
         return "Hello World!"
     }
 
-    @GetMapping("/example/html")
+    @GetMapping(value = ["/example/html"], produces = ["text/html"])
+    @ResponseBody
     fun exampleHtml(): String {
         return renderProductTable()
     }