Elasticsearch version: 5.0.0 (3ed4ff0)

JVM version: OpenJDK 64-Bit Server VM (build 25.74-b02, mixed mode)

OS version: Linux 4.1.20-1-lts

Description of the problem including expected versus actual behavior: When sorting on a single field and passing it in a list you get: Failed to derive xcontent error

Steps to reproduce:

1. curl -X PUT localhost:9200/i/t/42 -d '{"number": 123}'
2. curl localhost:9200/_search -d '{"sort": ["number"]}'