Use POST when service account token name is not present

[carlosdelest]

When a service account token is created, and no name is specified, the method to use is POST instead of PUT. PUT returns a 405 message:

curl -u [redacted] -X PUT http://localhost:9200/_security/service/elastic/enterprise-search-server/credential/token

{"error":"Incorrect HTTP method for uri [/_security/service/elastic/enterprise-search-server/credential/token] and method [PUT], allowed: [POST]","status":405} 

While POST creates the service account token:

curl -u [redacted] -X POST http://localhost:9200/_security/service/elastic/enterprise-search-server/credential/token

{"created":true,"token":{"name":"token_bgy1gYQB5WUVQc9_TiIn","value":[redacted]}}

[picandocodigo]

¡Gracias por el PR Carlos!
This code is automatically generated from the ES REST specification. So I'm going to update the generator to fix the method and add your changes 👍

[picandocodigo]

Included in #1964