[API] Updates transform.preview_transform, adds body

Author: picandocodigo

elasticsearch-xpack/lib/elasticsearch/xpack/api/actions/transform/preview_transform.rb

@@ -22,20 +22,30 @@ module Transform
         module Actions
           # Previews a transform.
           #
+          # @option arguments [String] :transform_id The id of the transform to preview.
           # @option arguments [Hash] :headers Custom HTTP headers
-          # @option arguments [Hash] :body The definition for the transform to preview (*Required*)
+          # @option arguments [Hash] :body The definition for the transform to preview
           #
           # @see https://www.elastic.co/guide/en/elasticsearch/reference/7.x/preview-transform.html
           #
           def preview_transform(arguments = {})
-            raise ArgumentError, "Required argument 'body' missing" unless arguments[:body]
-
             headers = arguments.delete(:headers) || {}
 
             arguments = arguments.clone
 
-            method = Elasticsearch::API::HTTP_POST
-            path   = "_transform/_preview"
+            _transform_id = arguments.delete(:transform_id)
+
+            method = if arguments[:body]
+                       Elasticsearch::API::HTTP_POST
+                     else
+                       Elasticsearch::API::HTTP_GET
+                     end
+
+            path   = if _transform_id
+                       "_transform/#{Elasticsearch::API::Utils.__listify(_transform_id)}/_preview"
+                     else
+                       "_transform/_preview"
+                     end
             params = {}
 
             body = arguments[:body]