API: ETag/If-None-Match for Get single note

docs/api/v1.md

@@ -91,9 +91,11 @@ No valid authentication credentials supplied.
 | Parameter | Type | Description |
 |:------|:-----|:-----|
 | `id` | integer, required (path) | ID of the note to query. |
+| `If-None-Match` | HTTP header, optional | Use this in order to reduce transferred data size (see [HTTP ETag](https://en.wikipedia.org/wiki/HTTP_ETag)). You should use the value from the note's attribute `etag` or from the last request's HTTP response header `ETag`. | 1.2 |
 
 #### Response
 ##### 200 OK
+- **HTTP Header**: `ETag` (see [HTTP ETag](https://en.wikipedia.org/wiki/HTTP_ETag)). The value is identical to the note's attribute `etag` (see section [Note attributes](#note-attributes)).
 - **Body**: note (see section [Note attributes](#note-attributes)), example:
 ```js
 {

lib/Controller/NotesApiController.php

@@ -68,7 +68,10 @@ public function get(int $id, string $exclude = '') : JSONResponse {
 		return $this->helper->handleErrorResponse(function () use ($id, $exclude) {
 			$exclude = explode(',', $exclude);
 			$note = $this->service->get($this->helper->getUID(), $id);
-			return $this->helper->getNoteData($note, $exclude);
+			$noteData = $this->helper->getNoteData($note, $exclude);
+			return (new JSONResponse($noteData))
+				->setETag($noteData['etag'])
+			;
 		});
 	}