[API] Updates knn_search

Author: picandocodigo

elasticsearch-api/lib/elasticsearch/api/actions/knn_search.rb

@@ -51,11 +51,7 @@ def knn_search(arguments = {})
 
         _index = arguments.delete(:index)
 
-        method = if body
-                   Elasticsearch::API::HTTP_POST
-                 else
-                   Elasticsearch::API::HTTP_GET
-                 end
+        method = Elasticsearch::API::HTTP_GET
 
         path = "#{Utils.__listify(_index)}/_knn_search"
         params = Utils.process_params(arguments)