Scroll property in SearchRequest throws a System.InvalidCastException

[HyruleWarrior]

NEST/Elasticsearch.Net version: 5.6.0

Elasticsearch version: 5.6.0

Description of the problem including expected versus actual behavior: Scroll property in SearchRequest throws a System.InvalidCastException when trying to retrieve it. In GetQueryStringValue() in FluentRequestParameters.cs, it is trying to explicitly cast the Scroll value (a string) into a Time object. Time only has implicit casts in it right now, so this throws an exception.

Steps to reproduce:

1. Create a SearchRequest with the Scroll property set
2. Try to read this property
3. Observe that it throws an exception when trying to evaluate it

[russcam]

Thanks for opening @HyruleWarrior; this has been fixed and will be in the next release 👍