Expose etag through JSON responses for child elements

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/Db/Label.php

@@ -36,4 +36,8 @@ class Label extends RelationalEntity {
 		$this->addType('cardId', 'integer');
 		$this->addType('lastModified', 'integer');
 	}
+
+	public function getETag() {
+		return md5((string)$this->getLastModified());
+	}
 }

lib/Db/RelationalEntity.php

@@ -80,6 +80,9 @@ class RelationalEntity extends Entity implements \JsonSerializable {
 				$json[$property] = $value;
 			}
 		}
+		if ($reflection->hasMethod('getETag')) {
+			$json['ETag'] = $this->getETag();
+		}
 		return $json;
 	}