Mark keep_alive as optional in open_point_in_time API

[pquentin]

keep_alive is an optional parameter:

elasticsearch/server/src/main/java/org/elasticsearch/action/search/RestOpenPointInTimeAction.java

Line 50 in 30dd38b

openRequest.keepAlive(TimeValue.parseTimeValue(request.param("keep_alive"), null, "keep_alive"));

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[mayya-sharipova]

keep_alive is a required parameter, and null value will not pass validation.

I guess null is passed into TimeValue.parseTimeValue function because this funciton requires a supply of default value.

[pquentin]

Sorry for the noise! Closing.