Search should default to _all when no default index is set

[gmarz]

Currently, if a default index is set then:

client.Search<Foo>(s => s.MatchAll());

will execute the request to: /defaultindex/foo/_search

However, if a default index is NOT set, the request will go to just: /_search, effectively ignoring the type.

Instead it should go to: /_all/foo/_search.