Add Vary header for Hx-Requests

The HTMX docs recommend doing this to avoid issues with caches
returning partial results.

htmx-spring-boot/src/main/java/io/github/wimdeblauwe/htmx/spring/boot/mvc/HtmxHandlerInterceptor.java

@@ -8,6 +8,7 @@
 import java.lang.reflect.Method;
 
 import org.springframework.core.annotation.AnnotatedElementUtils;
+import org.springframework.http.HttpHeaders;
 import org.springframework.web.method.HandlerMethod;
 import org.springframework.web.servlet.HandlerInterceptor;
 
@@ -23,11 +24,16 @@ public boolean preHandle(HttpServletRequest request,
             Method method = ((HandlerMethod) handler).getMethod();
             setHxTrigger(response, method);
             setHxRefresh(response, method);
+            setVary(response, method);
         }
 
         return true;
     }
 
+    private void setVary(HttpServletResponse response, Method method) {
+        response.addHeader(HttpHeaders.VARY, HtmxRequestHeader.HX_REQUEST.getValue());
+    }
+
     private void setHxTrigger(HttpServletResponse response, Method method) {
         HxTrigger methodAnnotation = AnnotatedElementUtils.findMergedAnnotation(method, HxTrigger.class);
         if (methodAnnotation != null) {

htmx-spring-boot/src/test/java/io/github/wimdeblauwe/htmx/spring/boot/mvc/HtmxHandlerInterceptorTest.java

@@ -9,6 +9,7 @@
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.header;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+import static org.hamcrest.collection.IsIterableContainingInRelativeOrder.containsInRelativeOrder;
 
 @WebMvcTest(TestController.class)
 @WithMockUser
@@ -59,6 +60,13 @@ public void testAnnotationCompositionWithAliasFor() throws Exception {
                .andExpect(header().string("HX-Trigger", "updateTrigger"));
     }
 
+    @Test
+    public void testVary() throws Exception {
+        mockMvc.perform(get("/hx-vary"))
+               .andExpect(status().isOk())
+               .andExpect(header().stringValues("Vary", containsInRelativeOrder("HX-Request")));
+    }
+
     @Test
     public void testHxRefresh() throws Exception {
         mockMvc.perform(get("/hx-refresh"))

htmx-spring-boot/src/test/java/io/github/wimdeblauwe/htmx/spring/boot/mvc/TestController.java

@@ -58,4 +58,10 @@ public String hxRefresh() {
         return "";
     }
 
+    @GetMapping("/hx-vary")
+    @ResponseBody
+    public String hxVary() {
+        return "";
+    }
+
 }